Surface Water Availability and Temperature (SWAT): An Innovative Index for Remote Sensing of Drought Observation
Yuei‐An Liou, Minh-Tin Thai
Abstract
Crop water stress and drought status can be indicat-ed through soil moisture (SM). SM-related factors, such as water availability, vegetation state, and temperature, can be inferred by Normalized Difference Latent Heat Index (NDLI), Normalized Difference Vegetation Index (NDVI), and Land Surface Temper-ature (LST), respectively. This study presents a new Surface Wa-ter Availability and Temperature (SWAT) index that utilizes remotely-sensed data to monitor crop water stress and drought status. The Euclidean distance method was used to analyze the three-dimensional (3D) scatterplots of NDLI, NDVI, and LST to define the SWAT. Subsequently, the SWAT and two drought indices, including the Temperature-Vegetation Dryness Index (TVDI) and Temperature-Soil Moisture Dryness Index (TMDI), were used to determine the spatial signatures of droughts in Tai-wan from 2013 to 2020. These drought signatures were evaluated and compared against corresponding assessments from various indicators, including the in-situ SM, TVDI, TMDI, Crop Water Stress Index (CWSI), and Net Primary Productivity (NPP). The results show that the TVDI and TMDI were negatively correlated with the in-situ SM, with the highest correlation coefficient (r) of −0.53 and −0.67, respectively. The SWAT was found to be even more correlated to the SM (–0.79 ≤ r ≤ –0.49, p < 0.01) and exhib-ited more sensitive and stable than other single indices (NDVI, NDLI, and LST) and integrated indices (TVDI and TMDI). Ac-cording to the spatial observation, the SWAT index was closely related to CWSI (r = 0.77, p < 0.01). Moreover, the SWAT was in line with NPP (r = −0.79, p < 0.01) and more sensitive to drought than the TVDI and TMDI. Furthermore, the SWAT was highly correlated with the TVDI, TMDI, CWSI, and NPP in most agri-cultural regions in Taiwan. Overall, the SWAT is proven to be a satellite index effectively describing agricultural drought stress.