Analysis on driving force of runoff change in hill-plain-wetland area of Naoli River Basin

In order to prevent the increase of evaporation in hilly-plain-wetland area, decrease of runoff and degeneration of wetland, it is necessary to find the driving force of runoff change in hilly-plain-wetland area. In the present study, Naoli River Basin, a typical plain-hilly-wetland area, was selected as the research object. Data were from Baoan, Baoqing, Hongqi mountain and other hydrologic stations. The model of TOPMODEL was used for the study. During model validation, the absolute value of relative error of runoff of secondary flood was 2.73%-8.33%, the absolute value of flood peak flow was 2.49%-10.47%, and the determination coefficient of flood process line was 0.612-0.893, which indicated the rationality of the description of spatial distribution of rainfall, water shortage in unsaturated zone and spatial distribution of flow in soil by using the model. The influencing factors of runoff including hydrometeorology, rainfall-runoff relationship, cover type and dry-land crop species were analyzed. And the scenarios with different combination of influencing factors were assumed. The effects of runoff factors and their changes on hydrological cycle and runoff were analyzed by using hydrological model. The results showed that the change of regional precipitation was the key driving force of runoff change. Rainfall and runoff were generally in a decreasing trend. And the decrease of runoff depth was more obvious than that of precipitation. The other factors also played important roles in the runoff change besides the rainfall, and their influences increased gradually. Climate change was also one of the important driving forces of runoff change. Temperature, vapor pressure, sunlight intensity and wind speed increased significantly in summer and autumn from 1965 to 2014, while temperature and vapor pressure increased significantly in spring, and wind speed decreased during the spring. During the period of May to October in the years of 1965-2014, the basin evaporation did not always increased with temperature, sunlight intensity, vapor pressure and wind speed. During some periods, the basin evaporation decreased as the increase of temperature, sunlight intensity, vapor pressure and wind speed for some special cover types. Furthermore, the change of cover type in catchment area was also one of the important driving forces for runoff change. The transformation of unused land into dry land, dry land into paddy land and the adjustment of agricultural structure accelerated the evapotranspiration in the studied area, and then resulted in the fast deceleration of runoff depth and the abnormal response of runoff depth to precipitation. Based on the analysis above, precipitation, climate factors and land cover types showed complex effects on the temporal and spatial variations of evaporation, root zone water content, unsaturated zone water content and runoff.