Within‐Year Changes in Hydraulic Properties of a Shallow Entisol in Farmland and Forestland

Abstract

For regions with shallow soils underlain by fractured parent rocks, it is important to understand the characteristics of soil water retention and hydraulic conductivity. This knowledge could support the development of strategies to manage water for crops. A farmland and a forestland with thin Entisols, located in hilly central Sichuan on the Yangtze River's upper reaches, were chosen as experimental fields. A variety of parameters related to hydraulically effective macroporosity (pore radius >125 μm), the single‐porosity van Genuchten model, and the dual‐porosity bi‐exponential (BE) model for water retention characteristics were determined. Tension infiltration data showed that soil macropores were the main pores (accounting for 87.9–99.7%) contributing to fast drainage at all tested soil depths under both land uses in the rainy summer, in spite of their very low percentage of the total porosity (0.61–3.06%). The available soil water content, determined as the drainable soil textural porosity using the bi‐exponential model (θtxt‐BE), was found to be very low (0.032–0.091 m3 m−3) and negatively (P < 0.05) related to the initial soil water content at the time of sampling, resulting in high vulnerability of the crops grown on the farmland to drought. Plowing operations and periodical drying appeared to be the two main factors controlling the within‐year change in soil hydraulic properties. For agricultural soils showing a bimodal pore size distribution, the soil's capacity to hold crop‐available water in textural pores, which may vary with the time of year, rather than the total soil water content should be analyzed to inform decisions on irrigation operations including required amounts.