An improved optical trapezoid model to identify irrigation water ponds in the arid irrigation district using Google Earth Engine

In arid regions conducting flood irrigation, irrigation practice often results in the temporary formation of water ponds in the cropland, which plays a critical role in agricultural productivity. The prompt identification of irrigation water ponds in irrigation districts is important for the effective management of irrigation water. Utilizing the OPtical TRApezoid Model (OPTRAM) for soil moisture estimation, we have proposed an improved version of OPTRAM aimed to identify irrigation water ponds in irrigation district using Sentinel-2 data through Google Earth Engine platform. While the wet edge determined from OPTRAM refer to those with saturated status, irrigation ponds are usually oversaturated regions. Therefore, an additional threshold was added and calibrated to the model in accordance with the irrigated area to identify irrigation water ponds. The improved OPTRAM was applied in Hetao Irrigation District (HID) of Northwest China from 2016 to 2023, where autumn irrigation applied in late autumn after crop harvesting was considered. The identified distributions of autumn irrigation were validated with observations from field survey and statistical data, and were also compared with other remote sensing products. Results show that the proposed model is effective in identifying irrigation ponds. The overall accuracy is 0.90 based on the observations from field survey, with mean absolute relative errors for irrigated areas across sub-irrigation districts recorded as 20.55%, 8.10%, 12.83%, and 11.38%, respectively, when compared with statistical data. With regard to temporal and spatial distributions, autumn irrigated croplands are mainly concentrated in Jiefangzha sub-irrigation district while being scattered across other sub-irrigation districts, depicting an overall decreasing trend in the autumn irrigated area. In summary, the proposed model performed well in identifying irrigation ponds and can offer valuable support for irrigation management.