This paper describes a series of numerical experiments simulating the effect of large-scale irrigation on short-term changes of hydrology and climate. This is done through the use of a simple general circulation model, with a limited computational domain and idealized geography.
The soil at three latitude bands, namely 30°N–60°N, 0–30°N, and 15°S–15°N is initially saturated with moisture. The results from these experiments indicate that irrigation affects not only the distribution of evaporation but also that of large-scale precipitation. It is found that the anomalies of soil moisture created by irrigation of these respective latitude zones can persist for at least several months due to increased evaporation and precipitation. Furthermore, it the irrigated region is located under a rainbelt, precipitation in that rainbelt is enhanced. Conversely, if the irrigated region is not located under a rainbelt, much of the additional moisture is transported to a rainbelt outside this area. Thus the moist moisture anomaly for the 30°N–60°N cast which is located under the middle latitude rainbelt tends to persist longer than the corresponding anomaly for the 0–30°N case.
Although both the 30°N–60°N and 15°S–15°N latitude regions occur under rainbelts, the soil moisture anomaly for the 15°S–15°N case does not persist as long as it does for the 30°N–60°N case. This is because in the 15°S–15°N case, a much greater fraction of the increased precipitation is lost from the hydrologic cycle due to runoff there as compared with the 30°N-60°N case.
The above changes of the hydrological processes also cause corresponding changes of the thermal state of the atmosphere such as a cooling of the surface due to increased evaporation. This results in change of the mean zonal circulation through the thermal wind relationship. It is found that irrigation in the tropical region weakens the upward branch of the Hadley circulation in the vicinity of the tropical rainbelt.