Assessment of Surface Hydrological Connectivity in an Ungauged Multi-Lake System with a Combined Approach Using Geostatistics and Spaceborne SAR Observations

Abstract

Connectivity metrics for surface water are important for predicting floods and droughts, and improving water management for human use and ecological integrity at the landscape scale. The integrated use of synthetic aperture radar (SAR) observations and geostatistics approach can be useful for developing and quantifying these metrics and their changes, including geostatistical connectivity function (GCF), maximum distance of connection (MDC), surface water extent (SWE), and connection frequency. In this study, we conducted a geostatistical analysis based on 52 wet and dry binary state (i.e., water and non-water) rasters derived from Sentinel-1 A/B GRD products acquired from 2015 to 2019 for China’s Momoge National Nature Reserve to investigate applicability and dynamics of the hydrologic connectivity metrics in an ungauged (i.e., data such as flow and water level are scarce) multi-lake system. We found: (1) generally, the change of GCF in North–South and Northeast–Southwest directions was greater than that in the West–East and Northwest–Southeast directions; (2) MDC had a threshold effect, generally at most 25 km along the W–E, NW–SE and NE–SW directions, and at most 45 km along the N–S direction; (3) the flow paths between lakes are diverse, including channelized flow, diffusive overbank flow, over-road flow and “fill-and-merge”; (4) generally, the values of the three surface hydrological connectivity indicators (i.e., the MDC, the SWE, and the conneciton frequency) all increased from May to August, and decreased from August to October; (5) generally, the closer the distance between the lakes, the greater the connection frequency, but it is also affected by the dam and road barrier. The study demonstrates the usefulness of the geostatistical method combining Sentinel-1 SAR image analysis in quantifying surface hydrological connectivity in an ungagged area. This approach should be applicable for other geographical regions, in order help resource managers and policymakers identify changes in surface hydrological connectivity, as well as address potential impacts of these changes on water resources for human use and/or ecological integrity at the landscape level.

Integrating the JRC Monthly Water History Dataset and Geostatistical Analysis Approach to Quantify Surface Hydrological Connectivity Dynamics in an Ungauged Multi-Lake System

“…However, due to excessive water, Scirpus nipponicus and Scirpus planiculmis in the Wulanzhaopao and Etoupao were submerged, preventing the Siberian cranes from obtaining food in these areas. The current results are consistent with the findings of our earlier study [15]. In addition, even in the wet year, the surface hydrological connectivity between the Nenjiang River and its floodplain in May was very poor, which may adversely affect the germination and growth of aquatic vegetation in the area.…”

“…Our results show that, during the dry year, the reduction rate of the GCF curve was slower along the W-E direction than that along the N-S direction, which is contrary to the situation during the normal and wet years ( Figure 3). As far as normal years are concerned, the current results of the reduction rate of the GCF curve using the JRC Monthly Water History dataset are consistent with the findings of our previous paper [15] using the Sentinel-1 dataset. Moreover, the minimum values of the MDC along the W-E and N-S directions both appeared in the dry year, at 22.4 km and 6.3 km, respectively, while the maximum values of the MDC along the above two directions both appeared in the wet year, at 50.7 km and 65.1 km, respectively (Figure 3).…”

“…During the normal year, the MDC along the W-E direction varied from 23.6 km to 36.4 km, while the MDC along the N-S direction varied from 10.0 km to 61.7 km (Figure 3). Whether in the W-E direction or in the N-S direction, the changes in the MDC in the current study were more dramatic than the findings of our previous paper [15]. The reason may be that the current paper only focused on one normal year, whereas the results of our previous paper were the average of five normal years.…”

“…Specifically, the surface hydrological connectivity directions between the Tao'er River and the study area and between the Nenjiang River and the study area are mainly west-east, while the surface hydrological connectivity directions between artificial ditches and the study area are mainly north-south. A simple example shows how to calculate the GCF along the W-E direction [15] (Figure 2). In addition, the script also creates a grid, where each of the connected objects (i.e., connectome) is identified by an integer from 1 to n. On this basis, we determined and mapped the top 10 largest connectomes in each month.…”

“…Specifically, this study aimed to (1) analyze changing trends in the geostatistical connectivity function and maximum distance of connection along the west-east (W-E) and north-south (N-S) directions during different hydrological years, (2) identify the spatiotemporal distribution and connection process of seasonal connected water bodies during different hydrological years, and (3) quantify the variation in surface water extent of the top 10 largest connectomes during different hydrological years. In our previous paper [15], we used Sentinel-1 synthetic aperture radar (SAR) data and a geostatistical analysis approach to quantitatively evaluate surface hydrological connectivity dynamics over five normal years in the same study area. Therefore, the results of the current manuscript can be regarded as a supplement to the previous paper.…”