SWAT-based assessment of spatiotemporal ecological flow deficit in the Yiluo River Basin

To address the ecological flow deficit caused by intensive human activities in the Yiluo River Basin, this study integrates the Soil and Water Assessment Tool (SWAT) with a locally modified Tennant method to systematically evaluate the spatiotemporal characteristics of ecological flow requirements and deficit patterns at the sub-basin scale. Two hydrological scenarios were constructed—a natural scenario representing undisturbed runoff processes and a baseline scenario incorporating actual anthropogenic disturbances such as reservoir operations, agricultural irrigation, and inter-basin water transfers—to quantify runoff dynamics and ecological flow thresholds under different ecological standards. The SWAT model produced reliable simulations: for the natural scenario, R 2 was 0.84 and 0.93 during the calibration and validation periods, with NSE of 0.76 and 0.90, respectively; for the baseline scenario, R 2 was 0.94 for both periods, with NSE of 0.92 and 0.94, respectively. The results indicate that: (1) baseline annual runoff decreased by approximately 40% compared with the natural state, with a reduction of up to 80% during the dry season (February–March); even in wet years (e.g., 2018 and 2021), observed runoff remained 35%–42% lower than natural runoff. (2) Ecological flow requirements exhibited pronounced spatiotemporal heterogeneity, ranging from a maximum of 30.2 m 3 /s in the wet season and 15.1 m 3 /s in the dry season at the mainstream outlet (Sub-basin 3) to a minimum of only 5.21 m 3 /s in an upstream tributary (Sub-basin 8); wet-season (June–October) demand was approximately 3–5 times that of the dry season (November–May). (3) Under the moderate Tennant standard, about 84% of the basin area met the basic ecological flow requirements; however, Sub-basins 4 and 6 had the highest deficit frequencies (approximately 63%), followed by Sub-basins 8 and 19 (approximately 50%), with deficits mainly concentrated in the wet season. (4) A priority-based ecological water replenishment framework was proposed, recommending targeted replenishment during the wet season (June–October) for high-frequency deficit areas. The target monthly replenishment volumes were: 1555.2 × 10 4 m 3 for the upper Luo River (Sub-basins 8, 10, and 11), 1166.4 × 10 4 m 3 for the upper Yi River (Sub-basins 15 and 19), 777.6 × 10 4 m 3 for the Lan River (Sub-basin 1), 648.0 × 10 4 m 3 for the Yongchang River (Sub-basin 5), and 1036.8 × 10 4 m 3 each for Sub-basins 4 and 6 (all values are theoretical estimates excluding conveyance losses). By coupling hydrological modeling with sub-basin-scale deficit diagnosis, this study provides an operational hydrological pre-screening basis for ecological water replenishment in the basin. However, it should be noted that the ecological flow defined in this study is a hydrological surrogate indicator and awaits biological validation .