Simulation of farmland runoff generation pattern considering the field ridge height under rainfall event conditions

Abstract: Runoff generation can be greatly varied in the events, such as the climate, catchment properties, and scale under the action of field ridges in farmland. It is crucial to better understand the spatiotemporal patterns of runoff generation for the process-based hydrological modelling. The evolution of the hydrologic cycle in the plain areas can also be strengthened to link the run-off behavior associated with the changes in environmental and land use conditions. However, it is still lacking on the relationship between the rainfall characteristics, ridge height, and the runoff generation in farmland. Taking the Haihe River Basin in northern China as the research area, this study aims to characterize the rainfall events using the Climate Prediction Center (CPC) Morphing Technique (MORPH, CMOPRH) remote sensing precipitation data products. Smith-Parlange infiltration method was used to construct a water flow model for the farmland accumulation, and then to simulate the rainfall and runoff generation process in the study areas. The results are as follows. 1) The total amount and duration of rainfall events in the study Plain were mainly less than 25 mm and 10 h in the field, accounting for 95% and 96% of the total rainfall events, respectively. In rainfall intensity, the light, moderate, and heavy rain accounted for 87%, 11%, and 1.9% of the total rainfall, while the rainstorm only accounted for 0.1% of the total rainfall. There was also a significant decrease in the total amount of rainfall and rainfall intensity from the northeast to the northwest. 2) The mean annual runoff was 5.47 billion m3 in the study area from 2008 to 2019, of which the farmland runoff was 2.18 billion m3, accounting for about 40% of the total runoff. Specifically, the yield of the Tuhaimajia River Basin was the highest, with an annual mean of 1.95 billion m3, whereas, that of the Ziya River Basin was the lowest, with an annual mean of 100 million m3. Four scenarios of no ridge, 10, 12, and 15 cm ridge were simulated under the rainfall conditions from 2008 to 2019, where the annual mean runoff generations of farmland were 6.24, 1.97, 1.31, and 0.5 billion m3/year, respectively. 3) Once the height of ridge was the constant, the rainfall duration and intensity were the key factors to affect the runoff generation of farmland. An inverse proportional function curve was fitted to classify the rainfall whether the runoff generation or not, where the rainfall duration and intensity were taken as the horizontal and vertical coordinate, respectively. For example, the runoff was formed at the ridge height of 10 cm, where the rainfall intensity was 10 mm/h, and the rainfall duration was more than 15.9 h. Once the rainfall intensity was 30 mm/h, and the rainfall duration was 2.4 h, the farmland runoff was formed in the study area. 4) The amount of farmland runoff decreased by 1.02 billion m3 in the study area in 2000, compared with 1979. This variation was attributed to both the evolution of the farmland area and the transition from no ridges to ridges in simulation. The amount of farmland runoff decreased by 510 million m3 in 2016, compared with 2000, which was mainly influenced by the decrease in the farmland area. Correspondingly, there was a significant decrease in the farmland runoff, due mainly to the decrease in the farmland area during the acceleration of urbanization in the study areas since 2000. Anyway, the quantitative analysis was offered to clarify the effects of rainfall characteristics and ridge height on farmland runoff generation. The finding can provide a strong reference to explore the causes of surface water resource attenuation in the Haihe plain area of northern.