Discussions

In this study, the effects of natural and anthropogenic heterogeneity on hydrological simulation were evaluated with a distributed biosphere hydrological model (DBHM) system. The DBHM was used to physically model the relationships between evaporation water demand, soil moisture deficit, and water availability. Precipitation variability is used to evaluate the effect of natural heterogeneity in hydrological cycle. Runoff simulation could be improved by taking into account the precipitation heterogeneity. However, the negative runoff contribution in dry region can only be simulated with the consideration of anthropogenic heterogeneity. The irrigation water withdrawals were estimated based on the model predicted soil moisture. The irrigation scheme does not take precipitation forecasts into account. The irrigation water withdraws from river way, and no reservoir is considered. Since the reservoir could store water for irrigation purpose, the irrigation water withdrawals might be underestimated. The irrigation scheme gives priority to upstream area. That is the upstream areas extract river water without considering the needs of downstream areas. That does not agree with integrated water management in the Yellow River basin, which is based on the water allocation rule along the river main stem. The irrigation scheme is based on the assumption that irrigation water is extracted from riverway and for the crop water use. The direct use of groundwater is not considered because of the data unavailability. The localized water use and water waste such as water consumption in endoric lake are not taken into account. However, our results indicate the method gives a reasonable approximation to the overall impact of irrigation in terms of large behavior of the hydrological system.

The study indicates that both natural and anthropogenic heterogeneity are important factors in the hydrological simulations. The precipitation variability and anthropogenic irrigation affect large scale distributed hydrological pattern in different way. Runoff, especially, surface runoff will increase over whole river basin with the consideration of the precipitation variability within grid cells. Anthropogenic heterogeneity caused by irrigation processes will increase evaporation and possibly educe negative runoff in intensively cultivated area. The effects of anthropogenic heterogeneity is localized, centralized and related to human activity intensity.