Spatiotemporal Evolution Characteristics and Driving Mechanism of Water Budget in the Sanjiang Plain

The water supply–demand pattern directly affects the stability of ecosystems and the level of agricultural sustainable development. Based on the theoretical framework of water supply–demand ecosystem services, this study took 1990–2020 as the research period, and integrated the water yield module of the InVEST model, urban stormwater retention module, standard deviational ellipse method, and spatial interpolation to accurately calculate regional water yield, water demand, water supply–demand ratio, and gravity center shift. The aim was to reveal the spatiotemporal evolution and mechanism of water budget in the Sanjiang Plain under large-scale agricultural expansion. The results showed the following: (1) From 1990 to 2020, the annual water yield in the Sanjiang Plain fluctuated from 12.16 billion m3/yr to 23.28 billion m3/yr, with a spatial pattern of “high in the northwest and southeast, low in the central Songhua River”, and high-value areas were highly coincident with natural ecological land. (2) The multi-year average water demand was approximately 13.0 billion m3/yr, experiencing three phases, “slow growth–rapid rise–stable slowdown”, showing a distribution characteristic of “concentrated along rivers and high in urban areas”. (3) Water surplus and deficit areas maintained a rigid characteristic of “highland supply, lowland consumption” for a long time. The gravity center of deficit areas rotated counterclockwise from 132.35° E, 46.55° N to 132.58° E, 46.78° N, with an annual migration rate of 10–15 km/a. The gravity center of surplus areas rotated clockwise from 133.22° E, 47.45° N to 132.85° E, 46.92° N, with an annual migration rate of 15–20 km/a. (4) From 1990 to 2015, the regional water supply–demand pattern remained relatively stable. However, a pronounced transition occurred during 2015–2020, when surplus areas sharply decreased from 49% to 27%, while conflict zones expanded rapidly, indicating an evident deterioration of regional water security.