## 1 Introduction  
Ecological water requirements (EWRs) refer to the total water amount to sustain watershed-scale water balance and maintain natural functions of ecosystems at an appropriate spatiotemporal scale [1, 2, 3, 4]. This notion has been applied to terrestrial and aquatic ecosystems and is of broad significance in arid and semi-arid areas where the water demand and supply are severely imbalanced [5, 6]. EWRs are generally sorted into EWRs of forest, grassland, urban and wetland, etc. [7, 8]. At basin scale, EWRs are primarily sorted into instream (aquatic) and off-stream (terrestrial) EWRs, i.e., from inside the river and from outside the channel, respectively [9; 10; 11; 12; 13]. From these perspectives, the determination of instream and off-stream EWRs is related to the significant interaction between water and the surrounding ecosystem, especially in fragile areas (e.g., arid and semi-arid river basins). Hence, there is an urgent need to ascertain the instream and off-stream EWRs and their water competition in arid and semi-arid river basins. Generally, the instream EWRs are the sum of annual river flow needed by aquatic ecosystems, while off-stream EWRs are the sum of annual actual evapotranspiration mostly needed by vegetation, including vegetation transpiration, interception evaporation, ponded detention evaporation, litter detention evaporation and soil evaporation at basin scale [14; 15]. The instream and off-stream EWRs are initially and mainly derived from precipitation, then partitioned into runoff (maintaining instream EWRs) and evapotranspiration (maintaining off-stream EWRs), respectively. Numerous studies have investigated the calculation methods of instream EWRs, including hydrological, hydraulic, habitat evaluation and holistic methods [16; 17; 18; 19; 20]. However, extremely low biodiversity and seasonal changes in aquatic ecosystems reduce the availability of data (particularly river ecosystem data) in arid and semi-arid basins, as these factors complicate monitoring efforts and ecosystem stability [21; 22; 23]. Consequently, hydraulic, habitat evaluation and holistic methods provide unsuitable results in these water-limited areas due to the lack of sufficient hydrological data (e.g., flow measurements) and ecosystem data in such regions. Additionally, most hydrological methods, requiring less data and suitable for semi-arid regions, fail to reflect seasonal variations in annual ecological water estimates, necessitating refinement [24]. The off-stream EWRs, which briefly refer to the EWRs of natural and artificially introduced vegetation water requirements in low-intensity human use basins, are evaluated and calculated by phreate evaporation, area-quota, water balance, and Penman-Monteith methods [25; 26; 27; 28; 29; 30]. However, precise evaluation of the EWRs is still not fully understood, owing to the disregard of physiological and ecological processes of vegetation. General hydrological models merely take vegetation growth as a series of static parameters to quantitatively determine the effect of vegetation on hydrological processes. The structure of these models does not fully consider the interaction between vegetation variation and critical hydrological elements including runoff and actual evapotranspiration. At the basin scale, evapotranspiration and runoff compete for limited water resources, making the balance between instream and off-stream EWRs crucial in arid and semi-arid basins. In areas with frequent flow interruptions, prioritizing off-stream ecological needs, especially for terrestrial ecosystems, becomes more important than merely focusing on aquatic ecosystems [31; 32; 33; 34; 35]. Therefore, the competitive relationship between instream EWRs and off-stream EWRs cannot be ignored in arid and semi-arid basins. Widespread vegetation changes were observed due to climate change and afforestation during the last several decades across the globe [36; 37; 35].