Historical Trends in snowmelt-dependent irrigation

Abstract:

Snowpack acts as a natural reservoir that holds water until it’s needed later in the season. Snowmelt that runs off as surface water is an important source of irrigation water in many regions, but is under threat in a warming climate. Here we examine historical trends in snowmelt runoff, rainfall runoff, and water demands across 33 snowmelt-dependent basins (mostly in Central Asia, the western US, and the southern Andes).

We use the TerraClimate dataset to quantify monthly surface water runoff and the portion of that runoff that is snowmelt (1985-2015). Crop-specific monthly irrigated water demands are estimated using the Global Crop Water Model for 26 crops scaled by harvested area from the Food and Agriculture Organization. The Terraclimate data are validated by comparing estimates of snow water equivalent (SWE) with a new product from the European Space Agency Climate Change Initiative that assimilates passive and active satellite data estimates of SWE. We calculate and map the supply and demand-side trends, considering changes to both the amount and timing.

We find that snowmelt runoff is a large fraction of total surface water runoff but that fraction has declined over time. We also find that snowmelt is occurring earlier in the year. At the same time, the demand for irrigation water has also increased. Consequently, an increase in demand, paired with a shift in the timing of snowmelt and subsequent decreases in surface water runoff, have led to increases in the need for alternative water sources to meet irrigation demand. Changes in snowmelt may therefore already be affecting agricultural production in some areas of the world.