Introducing an Irrigation Scheme to a Regional Climate Model: A Case Study over West Africa

https://journals.ametsoc.org/jcli/article/27/15/5708/34769/Introducing-an-Irrigation-Scheme-to-a-RegionalThis article presents a new irrigation scheme and biome to the dynamic vegetation model, Integrated Biosphere Simulator (IBIS), coupled to Regional ClimateModel version 3 (RegCM3-IBIS). The new land cover allows for only the plant functional type, crop, to exist in an irrigated grid-cell. Irrigation water (i.e. negative runoff) is applied until the soil root zone reaches relative field capacity. The new scheme allows for irrigation scheduling (i.e. when to apply water) and for the user to determine the crop to be grown. Initial simulations show a large sensitivity of the scheme to soil texture types, how the water is applied, and the climatic conditions over the region.

Application of the new scheme is tested over West Africa, specifically Mali and Niger, to simulate the potential irrigation of the Niger River. A realistic representation of irrigation of the Niger River is performed by constraining the land irrigated by the annual flow of the Niger River and the amount of arable land in the region as reported by FAO. A thirty-year simulation including irrigated cropland is compared to a thirty year simulation that is identical but with no irrigation of the Niger. Results indicate a significant greening of the irrigated land as evapotranspiration over the crop fields largely increases—mostly via increases in transpiration from plant growth. The increase in the evapotranspiration, or latent heat flux (by 65-150 W/m2), causes a significant decrease in the sensible heat flux while surface temperatures cool on average by nearly 5°C. This cooling is felt downwind where average daily temperatures outside the irrigation are reduced by 0.5-1.0°C. Likewise, large increases in two meter specific humidity (which will be referred to as QA) are experienced across the irrigated cropland (on the order of 5 g/kg) but also extend further north and east, reecting the prevailing surface southwesterlies. Changes (decreases) in rainfall are found only over the irrigated lands of west Mali. The decrease in rainfall can be explained by the large surface cooling and collapse of the boundary layer (by approximately 500m). Both lead to a reduction in the triggering of convection as the convective inhibition, or negative buoyant energy is never breached. Nevertheless, the new scheme and land cover allows for a novel line of research that can accurately reflect the effects of irrigation on climate and the surrounding environment using a dynamic vegetation model coupled to a regional climate model.