A method for identifying three types of drought transmission considering spatiotemporal correlation

Abstract

The present invention discloses a method for identifying the transmission of three types of droughts considering spatiotemporal association, comprising the following steps: step S1: constructing a standardized drought index for monitoring drought processes based on normal distribution transformation; step S2: extracting drought events using a three-dimensional spatiotemporal clustering identification method; step S3: identifying two types of drought events that have been transmitted using a spatiotemporal matching method for two types of drought events; step S4: identifying three types of drought events that have been transmitted using a spatiotemporal matching method for three types of drought events. The present invention further determines whether the propagation directions of drought event pairs are consistent, increases the possibility that the two types of drought events have a causal relationship, and then develops a transmission identification method for three types of drought events, effectively identifying three types of drought event pairs that have a causal relationship, and overcomes the limitations of traditional methods that are difficult to apply to larger spatiotemporal scales and cannot identify the transmission of three types of drought events.

Description

Technical Field

[0001] The invention relates to the technical field of drought identification, and in particular to a method for identifying three types of drought transmission by considering temporal and spatial correlation. Background Art

[0002] Drought is a global natural disaster. Based on the impact of drought, the drought categories generally recognized at home and abroad include meteorological drought, hydrological drought, agricultural drought and socio-economic drought. In recent years, many scholars have divided groundwater drought from hydrological drought to study groundwater level anomalies caused by multiple factors. With the increasing frequency and severity of drought, more and more hydrologists and ecologists have begun to pay attention to the ecological effects caused by drought (such as tree death, nutrient material circulation, etc.), and the concept of ecological drought has emerged.

[0003] The response process between different types of drought events is actually the transmission process of water shortage signals, which contain characteristic information such as intensity, severity and affected area. Therefore, extracting these transmission characteristics not only helps to deepen the understanding of the mechanism of drought disasters, but also is the basis for building a drought early warning system. In the past 20 years, the transmission of meteorological drought events to hydrological drought or agricultural drought has become a hot topic in the field of drought research, and many identification methods for drought event transmission have been developed. These methods mainly include two categories: the first category is the hydrological model or crop model method based on the drought event transmission process, and the second category is the statistical method based on the characteristics of drought events. Among them, the hydrological model / crop model method can effectively reflect the transmission mechanism of drought from the perspective of the transmission process, but this type of method includes a large number of parameter adjustment processes in the model training process, and it is difficult to apply to a larger time and space scale. Statistical methods do not need to set boundary conditions and are not limited by time and space. They are the main methods in drought transmission research, including cross wavelet method, multivariate joint distribution method, Granger causal analysis method, correlation analysis method, etc. However, when studying the transmission of drought events, most statistical methods are based on the run identification theory, using the time series of the mean of the drought index of the study area or its sub-regions for analysis. In other words, this method only considers the temporal correlation between different drought events. It should be emphasized that the impact range is an important attribute of drought events. If the spatial connection between the two types of drought events is ignored, it is very likely to misjudge the transmission of drought events in a larger spatial range. Summary of the invention

[0004] To solve the above problems, the present invention proposes a three-type drought transmission identification method considering spatiotemporal correlation, and establishes a transmission relationship model between meteorological drought events and hydrological drought events in the Yellow River Basin. This not only lays the foundation for the study of the response relationship between multi-type drought events, but is also an urgent problem to be solved in the process of building a drought early warning platform.

[0005] To achieve the above object, the present invention provides the following technical solution: a method for identifying three types of drought transmission considering spatiotemporal correlation, comprising the following steps:

[0006] Step S1: constructing a standardized drought index for monitoring drought processes based on normal distribution transformation;

[0007] Step S2: extracting drought events using a three-dimensional spatiotemporal clustering recognition method;

[0008] Step S3: using a spatiotemporal matching method of two types of drought events to identify two types of drought events that have been transmitted;

[0009] Step S4: using the spatiotemporal matching method of the three types of drought events to identify the three types of drought events that have occurred.

[0010] Preferably, the step S1 specifically follows the following steps:

[0011] Step S11: Time series x from January to December 1i Perform normalization to obtain a new sequence x 2i , 1 and 0 are replaced by 0.999 and 0.001 respectively to facilitate fitting the distribution, that is:

[0012]

[0013] In formula (1): x 1i is the original time series, x 2i is the normalized time series;

[0014] Step S12: Select three commonly used distributions, including Gamma distribution, log-logistic distribution and P-III distribution, and 2i Perform fitting and obtain the probability density equation f x2i (t) and use the Akaike information criterion, i.e., the AIC criterion, to select the best fitting distribution;

[0015] Step S13: Integrate the probability density function of the optimal distribution function to obtain the corresponding cumulative distribution F x2i (t); namely:

[0016]

[0017] In formula (2): f x2i (t) is the probability density function, F x2i (t) is the corresponding cumulative distribution function;

[0018] Step S14: x2i(t) is subjected to inverse normalization transformation to obtain the standardized drought index SDI, that is:

[0019] SDI = Φ -1 (F x2i ) (3);

[0020] In formula (3), Φ -1 is the inverse normalization transformation, SDI is the standardized drought index, and F x2i is the cumulative distribution obtained in step S13.

[0021] Preferably, in the said step S2, the steps of the three-dimensional spatio-temporal clustering recognition method for extracting drought events include spatial recognition of drought patches, spatio-temporal connection of drought patches, and extraction of drought event characteristics. The drought characteristics include drought intensity, drought duration, affected area, and migration direction. The calculation formula of drought intensity is:

[0022]

[0023] S n (i, j, k) = SDI n (i, j, k) × area n (i, j, k) × 1·month (5)

[0024] In formulas (4) and (5), DS n represents the drought intensity of the nth drought event; S n (i,j,k) represents the intensity value of the nth drought event; area n (i,j,k) is the patch area of the nth drought event; SDI n (i,j,k) is the drought index value of the nth drought event; i is the latitude value of each drought patch; j is the longitude value of each drought patch; k is the occurrence time of the drought;

[0025] The drought duration is the total time experienced by a drought event from start to end; the affected area of the drought is the projected area of all drought patches in the study area during the drought; the migration direction DO of the drought is the vector angle formed by the position mean of the intensity centroid in the latter half period and the position mean of the intensity centroid in the former half period. When -22.5° < DO ≤ 22.5°, it is migrating eastward; when 22.5° < DO ≤ 67.5°, it is migrating northeastward; when 67.5° < DO ≤ 112.5°, it is migrating northward; when 112.5° < DO ≤ 157.5°, it is migrating northwestward; when 157.5° < DO ≤ 202.5°, it is migrating westward; when 202.5° < DO ≤ 246.5°, it is migrating southwestward; when 246.5° < DO ≤ 291.5°, it is migrating southward; when 291.5° < DO ≤ 336.5°, it is migrating southeastward.

[0026] Preferably, the step S3 specifically follows the following steps:

[0027] Step S31: Based on the drought event identification results of step S2, arrange the drought events in the order of their occurrence to construct a drought event pair matrix, where each element in the matrix represents a set of drought event pairs to be identified; extract m drought events M and n drought events E respectively, and arrange them in the order of their occurrence to construct an m×n matrix X m,n , each element in the matrix represents a set of M-E drought event pairs to be identified;

[0028] Step S32: Identify whether the drought event pairs represented by each element in the matrix overlap in time, process the start and end time of the i-th drought event M and the j-th drought event E according to formula (6), and calculate the matrix X m,n Each element is iterated one by one to identify whether there is any time overlap and set to 1 or 0 respectively.

[0029]

[0030] In formula (5), 1 and 0 respectively indicate whether there is or is not a temporal overlap between the i-th drought event M and the j-th drought event E. If there is an overlap, then X i,j The value is 1, otherwise it is 0; MBT i and EBT j MET represents the start time of the i-th drought event M and the j-th drought event E respectively; i and EET j They represent the end time of the i-th drought event M and the j-th drought event E respectively;

[0031] Step S33: Identify whether the drought event pairs represented by each element in the matrix overlap in space; perform a second judgment on the M-E drought event pairs whose elements in the matrix X are 1, and judge whether the two types of drought events overlap in space according to equations (7) and (8). If the overlapping area is less than β or there is no overlap, replace 1 with 0;

[0032]

[0033] β=max(1.6%·A Nwc , min(MDA i ,EDA j ) b) (8):

[0034] In formula (7) and (8), MDA i and EDA jrepresent the impact range of the i-th drought event M and the j-th drought event E respectively; A NWC represents the total area of ​​the Northwest region; b is 15%;

[0035] Step S34: Perform a third judgment on the M-E drought event pairs whose elements in the matrix X are 1. According to formula (9), determine whether the propagation directions of the drought event pairs represented by each element in the matrix X are consistent. If the angle between the propagation directions of the two types of drought events is greater than 90°, the matching fails, and 1 is replaced by 0. The calculation formula is as follows:

[0036]

[0037] Where, MDO i and EDO j They represent the migration directions of the i-th drought event M and the j-th drought event E respectively;

[0038] Step S35: Classify the successfully matched drought event pairs ME; the pairing types include one drought event M triggering one drought event E (OTO), multiple drought events M triggering one drought event E (MTO), one drought event M triggering multiple drought events E (OTM) and multiple drought events M triggering multiple drought events E (MTM).

[0039] Preferably, in step S4, it is assumed that there are three types of drought, namely M, G and E, and both M and G can trigger E. At this time, the spatiotemporal matching method of the three types of drought events has two steps:

[0040] Step S41: Based on step S3, the drought event pairs ME and GE are identified respectively, with the horizontal axis being the drought event number of E and the vertical axis being the drought event numbers of M and G. Secondary matching is performed on the two drought event pairs ME and GE, that is, the two successfully matched drought event patches are simultaneously projected onto the coordinate axis of E, and the overlapping part is the E event jointly triggered by the M event and the G event;

[0041] Step S42: Extract drought characteristics of various drought events in drought event pairs; E events triggered by M events and G events include three types, namely, corresponding relationship, inclusion relationship and intersection relationship.

[0042] Preferably, in step S42, the method for extracting drought characteristics of drought events in various matching relationships is:

[0043] ① Corresponding relationship: The second E event is jointly affected by the first M event and the first to fourth G events. At this time, the total duration of the G event is the time from the beginning of the first to the end of the four G events. The total affected area is the projected area of ​​the G event in the study area within the duration, and the total intensity is the sum of the intensities of the G events within the duration.

[0044] ② Inclusion relationship: The 5th to 6th C event is jointly affected by the 2nd G event and the 2nd to 4th M event. At this time, the total duration of the M event is the difference between the latest end time of the 2nd to 4th M event and the start time of the 2nd M event; the total duration of the G event is the difference between the latest end time of the 5th to 6th E event and the start time of the 2nd M event; the total duration of the E event is the difference between the latest end time of the 5th to 6th E event and the start time of the 5th E event; the total affected area is the projected area of ​​various drought events in the duration in the study area, and the total intensity is the sum of the intensities of various drought events in the duration;

[0045] ③ Intersection relationship: The 12th to 14th E events are jointly affected by the 3rd to 5th G events and the 5th M event. At this time, the total duration of M events is the difference between the latest end time of the 12th to 14th E events and the start time of the 5th M event; the total duration of G events is the difference between the latest end time of the 3rd to 4th G events and the start time of the 5th M event; the total duration of E events is the difference between the latest end time of the 12th to 14th E events and the start time of the 12th E event; the total affected area is the projected area of ​​various types of drought events within the duration in the study area, and the total intensity is the sum of the intensities of various types of drought events within the duration.

[0046] Compared with the prior art, the present invention has the following beneficial effects:

[0047] The present invention increases the possibility that two types of drought events have a causal relationship by judging whether the propagation directions of the drought event pairs are consistent, and further develops a transmission identification method for three types of drought events, which effectively identifies the three types of drought event pairs with a causal relationship, overcoming the limitations of traditional methods that are difficult to apply to larger temporal and spatial scales and cannot identify the transmission of the three types of drought events. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Figure 1 The time-varying characteristics of (a) area and (b) intensity and the characteristics of different migration directions of ecological drought of the present invention;

[0049] Figure 2 The time-varying characteristics of (a) area and (b) intensity and the characteristics of different migration directions of the meteorological drought of the present invention;

[0050] Figure 3 The present invention provides the time-varying characteristics of (a) area and (b) intensity of groundwater drought and the characteristics of different migration directions;

[0051] Figure 4 is the matching type of the meteorological-ecological drought event pair of the present invention;

[0052] Figure 5 (a) is the meteorological-ecological drought event pair of the present invention, Figure 5 (b) for the groundwater-ecological drought event of the present invention and Figure 5 (c) Identification of meteorological-groundwater-ecological drought events;

[0053] Figure 6 The triggering types and their central spatial distribution of ecological drought events in 1982–2000 and 2001–2020 of this invention;

[0054] Figure 7 It is the matching type of the three types of drought events of the present invention;

[0055] Figure 8 It is the duration, intensity and area of ​​ecological drought caused by different factors in the present invention. DETAILED DESCRIPTION

[0056] The following will be combined with the drawings of the present invention in the embodiments of the present invention to clearly and completely describe the technical solutions of the present invention in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0057] The present invention provides a method for identifying three types of drought transmission considering temporal and spatial correlations;

[0058] S1: Construction of a standardized drought index for monitoring meteorological drought, groundwater drought and ecological drought processes based on normal distribution transformation

[0059] S2: Using three-dimensional spatiotemporal clustering recognition method to extract meteorological drought events, groundwater events and ecological drought events;

[0060] S3: The spatiotemporal matching method of two types of drought events is used to identify meteorological-ecological drought event pairs and groundwater-ecological drought event pairs with causal relationships.

[0061] S4: Using the spatiotemporal matching method of three types of drought events to identify the meteorological-groundwater-ecological drought event pairs with causal relationships

[0062] In step S1, the construction method of the standardized drought index has four steps:

[0063] S11: Normalize the precipitation series, groundwater storage series and ecological water shortage series from January to December to obtain new series. Replace 1 and 0 in the new series with 0.999 and 0.001 respectively to facilitate distribution fitting, that is:

[0064]

[0065] In the formula, x1i is the original time series, x 2i is the normalized time series.

[0066] S12: Select three commonly used distributions, including Gamma distribution, log-logistic distribution and P-III distribution, for x 2i Perform fitting and obtain the probability density equation f x2i (t), and use the Akaike information criterion (AIC criterion) to select the best fitting distribution;

[0067] S13: Integrate the probability density function of the optimal distribution function to obtain the corresponding cumulative distribution F x2i (t); namely:

[0068]

[0069] In formula (2): f x2i (t) is the probability density function, F x2i (t) is the corresponding cumulative distribution function.

[0070] S14: F x2i (t) Perform inverse normalization transformation to obtain the standardized drought index (SDI), namely:

[0071] SDI=Φ -1 (F x2i ) (3)

[0072] In formula (3), Φ -1 is the inverse normalization transformation, SDI is the standardized drought index, F x2i is the cumulative distribution obtained in step S13. The SDI in this case includes the standardized precipitation index (SPI) for meteorological drought, the standardized groundwater storage deviation index (SGSAI) for groundwater drought, and the standardized ecological water shortage index (SEWDI) for ecological drought.

[0073] In step S2, the step of extracting drought events by the three-dimensional spatiotemporal clustering recognition method includes spatial recognition of drought patches, spatiotemporal connection of drought patches, and extraction of drought event features. Drought features include drought severity, drought duration, affected area, and migration direction. The calculation formula of drought severity is:

[0074]

[0075] S n (i, j, k) = SDI n (i, j, k)×area n (i, j, k)×1·month (5)

[0076] In formulas (4) and (5), DS n represents the drought intensity of the nth drought event; S n (i, j, k) represents the intensity value of the nth drought event; area n (i, j, k) is the patch area of the nth drought event; SDI n (i, j, k) is the drought index value of the nth drought event; i is the latitude value of each drought patch; j is the longitude value of each drought patch; k is the occurrence time of the drought.

[0077] The drought duration is the total time experienced by a drought event from start to end; the affected area of the drought is the projected area of all drought patches during the drought in the study area; the migration direction (DO) of the drought is the vector angle formed by the mean position of the intensity centroid in the second half period and the mean position of the intensity centroid in the first half period. When -22.5° < DO ≤ 22.5°, it migrates eastward; when 22.5° < DO ≤ 67.5°, it migrates northeastward; when 67.5° < DO ≤ 112.5°, it migrates northward; when 112.5° < DO ≤ 157.5°, it migrates northwestward; when 157.5° < DO ≤ 202.5°, it migrates westward; when 202.5° < DO ≤ 246.5°, it migrates southwestward; when 246.5° < DO ≤ 291.5°, it migrates southward; when 291.5° < DO ≤ 336.5°, it migrates southeastward.

[0078] Based on step S2, meteorological drought events, groundwater drought events, and ecological drought events from 1982 to 2020 are extracted. As Figure 1 to Figure 3 shown: A total of 184 ecological drought events are identified. Drought events with high intensity, long duration, and large affected area mainly occurred before the 20th century and migrated northwestward ( Figure 1 ). 226 meteorological drought events are identified ( Figure 2 ), most of which occurred before the 21st century. Drought events with high intensity, long duration, and large affected area mainly occurred in the 1980s and early 1990s and mainly migrated in three directions: east, southeast, and northwest. 90 groundwater drought events are identified ( Figure 3 ). Groundwater drought events with high intensity, long duration, and large affected area mainly occurred from 1982 to 1990 and from 2011 to 2020 and mainly migrated in three directions: east, southeast, and south.

[0079] In step S3, there are four steps in the spatio-temporal matching method for two types of drought events. Taking the matching of meteorological drought - ecological drought event pairs as an example:

[0080] S31: Based on the drought event identification results of step S2, the drought events are arranged in the chronological order of occurrence to construct a drought event pair matrix, where each element in the matrix represents a set of drought event pairs to be identified.

[0081] In this case, m meteorological drought events and n ecological drought events are extracted and arranged in the chronological order of the drought events to construct an m×n matrix X m,n , each element in the matrix represents a set of meteorological-ecological drought event pairs to be identified.

[0082] S32: Identify whether the drought event pairs represented by each element in the matrix overlap in time;

[0083] According to formula (6), extract X i,j The corresponding information of the i-th meteorological drought event and the j-th ecological drought event is iterated one by one to identify whether there is temporal overlap between the elements in the matrix and set them to 1 and 0 respectively.

[0084]

[0085] In the formula, 1 and 0 respectively indicate whether there is or is not a temporal overlap between the i-th meteorological drought event and the j-th ecological drought event. If there is an overlap, then X i,j The value is 1, otherwise it is 0; MBT i and EBT j represent the start time of the i-th meteorological drought event and the j-th ecological drought event respectively; MET i and EET j They represent the end time of the i-th meteorological drought event and the j-th ecological drought event respectively.

[0086] S33: Identify whether the drought event pairs represented by each element in the matrix overlap in space. A second judgment is made for the meteorological-ecological drought event pairs whose elements in the matrix X are 1. According to equations (7) and (8), it is determined whether the two types of drought events overlap in space. If the overlapping area is less than β or there is no overlap, 1 is replaced by 0.

[0087]

[0088] β=max(1·6%·A NWC , min(MDA i ,EDA j )·b) (8)

[0089] Where, MDA i and EDA j A represents the impact area of ​​the i-th meteorological drought event and the j-th ecological drought event respectively; NWCrepresents the total area of ​​the Northwest region; b is 15%.

[0090] S34: Perform a third judgment on the M-E drought event pairs whose elements in the matrix X are 1. According to formula (9), determine whether the propagation directions of the drought event pairs represented by each element in the matrix X are consistent. If the angle between the propagation directions of the two types of drought events is greater than 90°, the matching fails and 1 is replaced by 0. The calculation formula is as follows:

[0091]

[0092] Where, MDO i and EDO j They represent the migration direction angles of the i-th meteorological drought event and the j-th ecological drought event respectively.

[0093] S35: Classify the successfully matched meteorological drought-ecological drought event pairs. The matching types include one meteorological drought event triggering one ecological drought event (OTO), multiple meteorological drought events triggering one ecological drought event (MTO), one meteorological drought event triggering multiple ecological drought events (OTM), and multiple meteorological drought events triggering multiple ecological drought events (MTM) ( Figure 4 ).

[0094] Based on step S3, 38 meteorological-ecological drought event pairs were obtained, of which 6, 2, 12 and 18 drought event pairs were one-to-one (OTO), one-to-many (OTM), many-to-one (MTO) and many-to-many (MTM), respectively. Most of the transmission types were more complex, with multiple meteorological drought events triggering one ecological drought event, or multiple meteorological drought events triggering multiple ecological drought events ( Figure 5 a). Similarly, there are 28 groundwater-ecological drought event pairs, and there are 3, 10, 6, and 9 drought event pairs for OTO, OTM, MTO, and MTM, respectively. Compared with meteorological drought events, groundwater drought events usually have a longer duration, so the transmission mode is often a situation where one or more groundwater drought events trigger multiple ecological drought events ( Figure 5 b).

[0095] In step S4, it is assumed that there are three types of drought, namely M, G and E, and both M and G can trigger E. At this time, the spatiotemporal matching method of the three types of drought events has two steps:

[0096] S41: Based on step S3, drought event pairs ME and GE are identified respectively, with the horizontal axis being the drought event number of E and the vertical axis being the drought event numbers of M and G. Secondary matching is performed on the two drought event pairs ME and GE, that is, the two successfully matched drought event patches are simultaneously projected onto the coordinate axis of E, and the overlapping part is the E event jointly triggered by the M event and the G event;

[0097] S42: Extract drought characteristics of various drought events in drought event pairs. The E events triggered by M events and G events include three types, namely, corresponding relationship, inclusion relationship and intersection relationship, such as Figure 7 The drought feature extraction method of drought events in various matching relationships is as follows:

[0098] ① Corresponding relationship: The second E event is jointly affected by the first M event and the first to fourth G events. At this time, the total duration of the G events is the time from the beginning of the first to the end of the first to fourth G events. The total affected area is the projected area of ​​the G events in the study area within the duration, and the total intensity is the sum of the intensities of the G events within the duration.

[0099] ② Inclusion relationship: The 5th to 6th C event is jointly affected by the 2nd G event and the 2nd to 4th M event. At this time, the total duration of the M event is the difference between the latest end time of the 2nd to 4th M event and the start time of the 2nd M event; the total duration of the G event is the difference between the latest end time of the 5th to 6th E event and the start time of the 2nd M event; the total duration of the E event is the difference between the latest end time of the 5th to 6th E event and the start time of the 5th E event. The total affected area is the projected area of ​​various drought events in the duration in the study area, and the total intensity is the sum of the intensities of various drought events in the duration.

[0100] ③ Intersection relationship: The 12th to 14th E events are jointly affected by the 3rd to 5th G events and the 5th M event. At this time, the total duration of the M event is the difference between the latest end time of the 12th to 14th E events and the start time of the 5th M event; the total duration of the G event is the difference between the latest end time of the 3rd to 4th G events and the start time of the 5th M event; the total duration of the E event is the difference between the latest end time of the 12th to 14th E events and the start time of the 12th E event. The total affected area is the projected area of ​​various drought events in the duration in the study area, and the total intensity is the sum of the intensities of various drought events in the duration.

[0101] Based on step 41, 31 meteorological-groundwater-ecological drought event pairs are obtained ( Figure 5 c), among which, ecological drought events that only matched meteorological drought, ecological drought events that only matched groundwater drought, ecological drought events that matched both, and ecological drought events that did not match accounted for 15.5%, 14.4%, 59.1%, and 11.0%, respectively. The drought centers of the 184 ecological drought events were spatially projected in two periods, 1982-2000 and 2001-2020, and different colors of points were used to represent different trigger types ( Figure 6). In general, ecological drought events caused only by meteorological drought events are mainly distributed in the Hexi Corridor, northern Qinghai and the border between Shaanxi and Gansu, while ecological drought events caused only by groundwater drought events are mainly distributed in southeastern Gansu, southeastern Qinghai and southern Shaanxi. In addition, the proportion of ecological drought events caused by other factors has increased significantly since the 21st century, mainly distributed in southeastern Gansu, southern Ningxia and central Shaanxi, where human activities have a greater impact, indicating that the disaster-causing factors of ecological drought events have become more complex since the 21st century.

[0102] Based on step 42, the characteristics of ecological drought events of each driving type are sorted out ( Figure 8 ), the duration, intensity and affected area of ​​ecological drought events caused by other factors were 8.0 months, 1.20×10 6 Month km 2 and 0.30×10 6 km 2 The mean duration, intensity and affected area of ​​ecological drought events that only matched groundwater drought events were 9.4 months, 1.24×10 6 Month km 2 and 0.36×10 6 km 2 The mean duration, intensity and affected area of ​​ecological drought events that only matched meteorological drought events were 4.4 months, 0.35×10 6 Month km 2 and 0.14×10 6 km 2 The mean duration, intensity, and affected area of ​​ecological drought events that matched meteorological drought events and groundwater drought events were 14.7 months, 2.85×10 6 Month km 2 and 0.67×10 6 km 2 It can be seen that the intensity, duration and affected area of ​​ecological drought events caused by both meteorological drought events and groundwater drought events are the largest, followed by those caused only by groundwater drought events and other factors, and the characteristic value of ecological drought events caused only by meteorological drought events is the smallest.

[0103] The spatiotemporal matching method of drought events can not only retain the characteristics of drought events extracted by traditional methods, such as duration and intensity information, but also provide area information of drought events by judging whether there is spatial superposition. On this basis, this study further judged whether the propagation direction of drought event pairs was consistent, increased the possibility that the two types of drought events had a causal relationship, and then developed a transmission identification method for three types of drought events, effectively identifying three types of drought event pairs with a causal relationship, overcoming the limitations of traditional methods that are difficult to apply to larger spatiotemporal scales and cannot identify the transmission of three types of drought events.

[0104] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for identifying three types of drought transmission considering spatiotemporal correlation, characterized in that: The following steps are involved: Step S1: constructing a standardized drought index for monitoring drought processes based on normal distribution transformation; Step S2: extracting drought events using a three-dimensional spatiotemporal clustering recognition method; Step S3: using a spatiotemporal matching method of two types of drought events to identify two types of drought events that have been transmitted; Step S4: using a spatiotemporal matching method of three types of drought events to identify the three types of drought events that have occurred; Step S31: Based on the drought event identification results of step S2, the drought events are arranged in the time sequence of occurrence to construct a drought event pair matrix, where each element in the matrix represents a set of drought event pairs to be identified; Extract m drought events M and n drought events E respectively, and arrange them in the chronological order of drought events to construct an m×n matrix Xm,n. Each element in the matrix represents a set of M-E drought event pairs to be identified. Step S32: Identify whether the drought event pairs represented by each element in the matrix overlap in time; Step S33: Identify whether the drought event pairs represented by each element in the matrix overlap in space; Step S34: Perform a third judgment on the drought event pairs M-E whose elements in the matrix X are 1, to determine whether the propagation directions of the drought event pairs represented by the elements in the matrix X are consistent; Step S35: classifying the MEs for the successfully matched drought events; The pairing types include one drought event M triggering one drought event E (OTO), multiple drought events M triggering one drought event E (MTO), one drought event M triggering multiple drought events E (OTM), and multiple drought events M triggering multiple drought events E (MTM); In step S4, it is assumed that there are three types of droughts, namely M, G and E, and both M and G can trigger E. At this time, the spatiotemporal matching method of the three types of drought events has two steps; Step S41: Based on step S3, the drought event pairs ME and GE are identified respectively, with the horizontal axis being the drought event number of E and the vertical axis being the drought event numbers of M and G. Secondary matching is performed on the two drought event pairs ME and GE, that is, the two successfully matched drought event patches are simultaneously projected onto the coordinate axis of E, and the overlapping part is the E event jointly triggered by the M event and the G event; Step S42: extract drought characteristics of various drought events in the drought event pair; the E event jointly triggered by the M event and the G event includes three types, namely, corresponding relationship, inclusion relationship and intersection relationship.

2. A method for identifying three types of drought transmission considering spatiotemporal correlation according to claim 1, characterized in that: The step S1 specifically comprises the following steps: Step S11: Time series x from January to December 1i Normalize and get the new sequence x 2i , 1 and 0 are replaced by 0.999 and 0.001 respectively to facilitate fitting the distribution, that is: In formula (1): x 1i is the original time series, x 2i is the normalized time series; Step S12: Select three commonly used distributions, including Gamma distribution, log-logistic distribution and P-III distribution, and 2i Perform fitting and obtain the probability density equation f x2i (t) and use the Akaike information criterion, i.e., the AIC criterion, to select the best fitting distribution; Step S13: Integrate the probability density function of the optimal distribution function to obtain the corresponding cumulative distribution function F x2i (x); namely: In formula (2): f x2i (t) is the probability density function, F x2i (x) is the corresponding cumulative distribution function; Step S14: x2i (t) Perform inverse normalization transformation to obtain the standardized drought index SDI, namely: SDI=Φ -1 (F x2i ) (3); In formula (3), Φ -1 is the inverse normalization transformation, SDI is the standardized drought index, F x2i is the cumulative distribution obtained in step S13.

3. A method for identifying three types of drought transmission considering spatiotemporal correlation according to claim 1, characterized in that: In step S2, the step of extracting drought events by the three-dimensional spatiotemporal clustering recognition method includes spatial recognition of drought patches, spatiotemporal connection of drought patches, and extraction of drought event features. Drought features include drought severity, drought duration, affected area, and migration direction. The calculation formula of drought severity is: S n (i,j,k)=SDI n (i,j,k)×area n (i,j,k)×1·month (5) In formula (4) and (5), DS n represents the drought severity of the nth drought event; S n (i,j,k) represents the intensity value of the nth drought event; area n (i, j, k) is the patch area of ​​the nth drought event; SDI n (i, j, k) is the drought index value of the nth drought event; i is the latitude value of each drought patch; j is the longitude value of each drought patch; k is the occurrence time of drought; The drought duration is the total time experienced by a drought event from its start to end; the affected area of the drought is the projected area of all drought patches in the study area during the drought; the migration direction DO of the drought is the vector angle formed by the mean position of the intensity centroid in the second half period and the mean position of the intensity centroid in the first half period. When -22.5° < DO ≤ 22.5°, it migrates eastward; when 22.5° < DO ≤ 67.5°, it migrates northeastward; when 67.5° < DO ≤ 112.5°, it migrates northward; when 112.5° < DO ≤ 157.5°, it migrates northwestward; when 157.5° < DO ≤ 202.5°, it migrates westward; when 202.5° < DO ≤ 246.5°, it migrates southwestward; when 246.5° < DO ≤ 291.5°, it migrates southward; when 291.5° < DO ≤ 336.5°, it migrates southeastward.

4. The method for identifying three types of drought transmission considering spatiotemporal correlation according to claim 1, characterized in that: The specific steps of step S3 are as follows: Step S31: Based on the drought event identification results of step S2, arrange them in the chronological order of the occurrence of drought events to construct a drought event pair matrix, where each element in the matrix represents a group of drought event pairs to be identified; Extract m drought events M and n drought events E respectively, and arrange them in the chronological order of the drought events to construct an m×n matrix X m,n , each element in the matrix represents a set of M-E drought event pairs to be identified; Step S32: Identify whether the drought event pairs represented by each element in the matrix overlap in time, process the start and end time of the i-th drought event M and the j-th drought event E according to formula (6), and calculate the matrix X m,n Each element is iterated one by one to identify whether there is any time overlap and set to 1 or 0 respectively. In formula (5), 1 and 0 respectively indicate whether there is or is not a temporal overlap between the i-th drought event M and the j-th drought event E. If there is an overlap, then X i,j The value is 1, otherwise it is 0; MBT i and EBT j MET represents the start time of the i-th drought event M and the j-th drought event E respectively; i and EET j They represent the end time of the i-th drought event M and the j-th drought event E respectively; Step S33: Identify whether there is a spatial overlap between the drought event pairs represented by each element in the matrix; make a second judgment on the M-E drought event pairs with the element value of 1 in matrix X. According to formulas (7) and (8), judge whether there is a spatial overlap between the two types of drought events. If the overlap area is less than β or there is no overlap, replace 1 with 0; β=max(1.6%·A NWC ,min(MDA i ,EDA j )·b) (8); In formula (7) and (8), MDA i and EDA j represent the impact range of the i-th drought event M and the j-th drought event E respectively; A NWC represents the total area of ​​the Northwest region; b is 15%; Step S34: Make a third judgment on the M-E drought event pairs with the element value of 1 in matrix X. Judge whether the propagation directions of the drought event pairs represented by each element in matrix X are consistent according to formula (9). If the included angle between the propagation directions of the two types of drought events is greater than 90°, the matching fails, and replace 1 with 0. The calculation formula is as follows: Where, MDO i and EDO j They represent the migration directions of the i-th drought event M and the j-th drought event E respectively; Step S35: Classify the successfully matched drought event pairs M-E; The pairing types include one drought event M triggering one drought event E (OTO), multiple drought events M triggering one drought event E (MTO), one drought event M triggering multiple drought events E (OTM), and multiple drought events M triggering multiple drought events E (MTM).

5. The method for identifying three types of drought transmission considering spatiotemporal correlation according to claim 1, characterized in that: In step S42, the drought feature extraction method for drought events in various matching relationships is as follows: ① Corresponding relationship: The second E event is jointly affected by the first M event and the first to fourth G events. At this time, the total duration of the G events is the time experienced from the start of the first to the end of the fourth G event among the first to fourth G events, the total affected area is the projected area of the G events in the study area during the duration, and the total intensity is the sum of the intensities of the G events during the duration; ② Inclusion relationship: The 5th to 6th C event is jointly affected by the 2nd G event and the 2nd to 4th M event. At this time, the total duration of the M event is the difference between the latest end time of the 2nd to 4th M event and the start time of the 2nd M event; the total duration of the G event is the difference between the latest end time of the 5th to 6th E event and the start time of the 2nd M event; the total duration of the E event is the difference between the latest end time of the 5th to 6th E event and the start time of the 5th E event; the total affected area is the projected area of ​​various drought events in the duration in the study area, and the total intensity is the sum of the intensities of various drought events in the duration; ③ Intersection relationship: The 12th to 14th E events are jointly affected by the 3rd to 5th G events and the 5th M event. At this time, the total duration of M events is the difference between the latest end time of the 12th to 14th E events and the start time of the 5th M event; the total duration of G events is the difference between the latest end time of the 3rd to 4th G events and the start time of the 5th M event; the total duration of E events is the difference between the latest end time of the 12th to 14th E events and the start time of the 12th E event; the total affected area is the projected area of ​​various types of drought events within the duration in the study area, and the total intensity is the sum of the intensities of various types of drought events within the duration.

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