Dynamic variation of soil moisture at different temporal scales in red soil sloping farmland under reverse-slope terrace

Abstract: In order to study the effects of reverse-slope terrace on soil moisture changes at different temporal scales and the changes of dry and wet states in red soil slope farmland, the monitoring of the soil moisture were carried out by time domain reflectometer in 2 sloping farmland in central Yunnan Province. One was the sloping farmland with contour reverse-slope terrace, and the other one was the original sloping farmland. Soil moisture at depths of 40, 60, 80 and 100 cm was measured by buried waveguide, and soil moisture at depths of 5 and 20 cm was measured by straight insert waveguide. The monitoring period in this study was from January 1st, 2016 to December 31st, 2017. During this period, soil moisture was measured every 10 days in the dry season (January to April), every 3 days from the dry season to rainy season (May), every day in the rainy season (June to October), and every 3 days from the rainy season to dry season (November to December). The results showed that after the reverse-slope terrace was used, the soil moisture at different depths was significantly improved at annual scale. Effect of reverse-slope terrace on soil moisture were more obvious in dry years at the annual scale. The variable coefficient of soil moisture was higher at the surface soil, reaching about 50%. At the monthly scale, soil moisture followed an obvious "S" shape characteristic in July, September and November along the soil profile. Specifically, the "S" was more pronounced in July. After reverse-slope terrace was laid in sloping farmland, the soil moisture content of all soil layers had been significantly increased in each period. In particular, the effect on increasing of soil moisture was more obvious in May when soil moisture was replenished and in November when soil moisture was decreased. The variation degree of the daily soil moisture content in sloping farmland was decreased with soil depth increasing. There was a extremely significant correlation between daily soil moisture content and daily rainfall at 5, 20 and 40 cm of sloping farmland (P<0.01). Meanwhile, there was also a significant correlation between daily soil moisture content and daily rainfall at 60 cm (P<0.05). However, significant correlation was not found between daily soil moisture content and rainfall at depths of 80 cm and 100 cm. With the increase of soil depth, the correlation coefficient between soil moisture content and rainfall decreased gradually. The average soil moisture conservation rates of reverse-slope terrace at 5, 20, 40, 60, 80 and 100 cm in sloping farmland were 15.22%, 15.25%, 16.91%, 15.60%, 16.50%, and 16.17%, respectively. The soil moisture conservation rates of reverse-slope terrace were different at different soil layers during monitoring period. Reverse-slope terrace lengthened the wet period of sloping farmland, and significantly increased the soil moisture in wet period and dry period. Soil moisture content of sloping farmland stayed at an ideal level for a long time after reverse-slope terrace was used, which had a significant effect on regulating soil moisture of sloping farmland. It also significantly increased the rainfall utilization rate of sloping farmland, which would be of great significance for solving the eco-hydrological drought problem of sloping farmland and improving the agricultural productivity of sloping farmland in mountainous areas.