In the inland closed area of Horqin Sandy Land, China, lakes are the most important source of water, and they maintain the regional hydrological balance and ecosystem health. Clarifying long-term continuous changes of inland lake surface area and water storage in the sandy land is thus of great significance to the management of water resources in arid and semi-arid areas. This study estimated changes in the surface area and water storage of small lakes in the sandy land during 1984–2021 using a multiple index threshold method and an empirical equation based on Shuttle Radar Topography Mission (SRTM) DEM (digital elevation model) data and Landsat 5/7/8 images. Hydro-meteorological variables were also incorporated to explore their potential relationship with changes in the lake elements. The lakes in the sandy land resemble stars or dots, with distinct inhomogeneity. Permanent lakes account for the majority of the total lake area, mostly distributed in the center and east of the study area, whereas most seasonal lakes are small water bodies, with surface areas of 0.1–0.5 km² and irregularly distributed. Lake surface area and water storage underwent frequent changes during the 38 years, and could be divided into three characteristic fluctuation phases. From 1984 to 1999, the lake surface area fluctuated greatly, and the water storage increased by approximately 0.3732 Gt, with an average expansion rate of 0.0572 Gt/y. Specifically, lakes with a surface area >2 km² primarily accounted for the expansion. From 1999 to 2004, the lake area shrank sharply to the lowest point, and the water storage capacity decreased by approximately 0.4003 Gt. From 2004 to 2021, the lake surface area and water storage tended to be stable. Annual fluctuations of lake surface area and water storage were mostly affected by precipitation and evapotranspiration, followed by vapor pressure deficit, wet day frequency, and temperature, which have significant periodicity and hysteresis.

Accurate evapotranspiration (ET) estimation is vital for the ecology and water resource management of arid and semiarid regions. This study focused on the Kolqin Sandy Land Inland Closed Basin. The simulation of energy flux data for the area was conducted by applying Landsat 8 data from 2018 to 2022 during the growing season, the Two-Source Energy Balance (TSEB), and the Mapping Evapotranspiration at High Resolution with Internalized Calibration (METRIC) models. The validation of both models was performed via eddy covariance systems and large-aperture scintillometers, and the applicability of both models was compared. At the ecosystem scale, the TSEB model exhibited excellent accuracy of latent heat flux simulation in dune and meadow ecosystems (R²>0.8, RMSE<45.74 W/m2, and MAPE<26%), while the METRIC model excelled in meadow ecosystems (R2 = 0.89, RMSE = 27.79 W/m2, and MAPE = 12%). In contrast, at the regional scale, the TSEB model outperformed METRIC for sensible heat flux simulation (R2: 0.65-0.68, RMSE: 22.83-43.28 W/m2, MAPE: 14%-17%). The TSEB model ranked evapotranspiration in the order of lakes>meadow wetlands>farmland>dunes, peaking in August and following a seasonal pattern of summer>spring>autumn. Additionally, evapotranspiration varied with LAI during the pre-middle growth period and responded significantly to △T in the late vegetation period.