Integrated assessment of Hadley Centre (HadCM2) climate change projections on agricultural productivity and irrigation water supply in the conterminous United States: I. Climate change scenarios and impacts on irrigation water supply simulated with the HU

In response to a congressional mandate, the US Global Change Research Program organized a National Assessment of Climate Change focusing on geographic regions (e.g. Alaska, Great Plains) and sectors (e.g. public health, agriculture, water resources). This paper describes methodology and results of a study by researchers at the Pacific Northwest National Laboratory contributing to the water sector analysis. The subsequent paper makes use of the water supply results to estimate the climate change impacts on irrigated agriculture. The vulnerability of water resources in the conterminous US to climate changes in 10-year periods centered on 2030 and 2095 as projected by the Hadley/United Kingdom Meteorological Office (UKMO) general circulation model (GCM; HadCM2) were modeled using the Hydrologic Unit Model for the United States (HUMUS). HUMUS, a biophysically based hydrology model, consists of a Geographical Information System (GIS) that provides data on soils, land use and climate to drive the Soil Water Assessment Tool (SWAT). The modeling was done at the scale of the eight-digit United States Geological Survey (USGS) Hydrologic Unit Area (HUA) of which there are 2101 in the conterminous US. Results are aggregated to the four- and two-digit (major water resource region, MWRR) scales for various purposes. Daily records of maximum/minimum temperature and precipitation (PPT) from 1961 to 1990 provided the baseline climate. Water yields (WY), used as a measure of water supply for irrigation, increases from the 1961–1990 baseline period over most of the US in 2030 and 2095. In 2030, WY increases in the western US and decreases in the central and southeast regions. Notably, WY increases by 139 mm (35%) from baseline in the Pacific Northwest. Driven by higher temperatures and reduced precipitation, WY is projected to decrease in the Lower Mississippi and Texas Gulf basins. The HadCM2 (2095) scenario projects a climate significantly wetter than baseline, resulting in water yield increases of 38% on average. Water yield increases are projected to be significant throughout the eastern US—39% in the Ohio basin, for example. Water yields increase significantly in the western US, as well—57 and 76% in the Upper and Lower Colorado, respectively. Climate change also affects the seasonality of the hydrologic cycle. Early snowmelt is induced in western basins, leading to dramatically increased water yields in late winter and early spring. The simulations were run at current (365 ppm) and elevated (560 ppm) atmospheric CO2 concentrations [CO2] to account for the potential impacts of the ‘CO2-fertilization’ effect. The effects of climate change scenario were considerably greater than those due to elevated [CO2] but the latter, overall, decreased losses and augmented increases in water yield.