Phreatic evaporation in bare and wheat land during freezing-thawing period of Huaibei Plain based on lysimeters experiments

Abstract: This paper was aimed to study phreatic evaporation in Huaibei Plain during freezing-thawing period. The data was collected from long-term experiments at Wudaogou Hydrological Experimental Station from 1990 to 2018. In the experiments, a total of 38 lysimeters were installed. About half of them were planted with winter wheat. The others were bare lands. The soils in lysimeters were typical local soils: undisturbed lime concretion black soil and yellow moist soil. Soil temperature and air temperature were measured. The phreatic evaporation was determined and its relationship with soil depth was fitted with non-linear regression equations. According to the air temperature, the freezing-thawing period in the Huaibei Plain was from December to the next February. During the period, the soil was characterized with freezing at night and thawing during the day. The freezing layer was thin. In bare land, the phreatic evaporation increased firstly with depth and then decreased. The maximum phreatic evaporation occurred in the depth range of 0.1-0.3 m in the both soils. When the depth was smaller than the 0.1 m, phreatic evaporation increased with depth while it decreased with the depth when the depth was higher than 0.3 m. The characteristics of phreatic evaporation in soil profile during the freezing-thawing period was different from that in the period. The change of phreatic evaporation in the wheat land was similar with that in the bare land. However, the phreatic evaporation in the wheat land was smaller than that in the bare land when the depth was smaller than 0.4 m in lime concretion black soil and smaller than 1.0 m in the yellow moist soil. It was because the capillary was probably cut off due to covering on soil surface caused by roots or stalk residues or tillage before sowing, which affected the transport of water along the capillary and caused small phreatic evaporation. There was a peak in the phreatic evaporation curves. Therefore, 3 forms of distribution functions were selected to fit the change of phreatic evaporation with depth. Meanwhile, popular phreatic evaporation equations were compared. The Gaussian function could yield the best simulation for the phreatic evaporation in the bare land with the determination coefficient higher than 0.9 and the root mean square error smaller than 0.1. During the freezing-thawing period, the maximum phreatic evaporation from the lime concretion black soil occurred at 0.08 m below ground surface but at 0.29 m in yellow moist soil below ground surface. For wheat lands, the quasi-Gauss function was the best for constructing phreatic evaporation simulation formula with the determination coefficient higher than 0.9. The maximums of phreatic evaporation from the lime concretion black soil and the yellow moist occurred on the soil surface and 0.23 m below the soil surface, respectively. The soil depth corresponding to maximum phreatic evaporation was smaller in the wheat land than bare land.