Effects of winter irrigation on soil hydro-thermal features in desert oasis farmland in arid area during freezing and thawing period

Abstract: Winter irrigation has proven to have positive effects on alleviating the "spring drought" and promoting the growth of crops. In seasonally frozen regions, a special rule of soil hydro-thermal movement is formed during the process of freezing and thawing. However, few studies have focused on the effects of winter irrigation on soil hydro-thermal dynamics in the farmland ecosystem of arid desert areas. Therefore, the aim of this study was to investigate the soil hydro-thermal features during the freezing-thawing period after winter irrigation with simulated soil column experiment. The experiment was conducted in marginal farmlands of oasis in middle Hexi Corridor region, Gansu, China. It was located in arid and semi-arid desert-oasis areas. Two types of soil, i.e. ploughed aeolian sandy soil (newly cultivated sandy cropland) and ploughed grey-brown desert soil (old oasis cropland), were backfilled into the soil columns. Each type of soil had 3 repetitions. Winter irrigation (160 mm) was conducted on October 31, 2016. Then the ECH2O soil moisture-temperature sensors were employed to measure soil moisture content and soil temperature at 10, 20, 40, 60, 80, 100, 120, 140, 160 and 180 cm soil depth from November 11, 2016 to April 11, 2017. In addition, the processes of soil water leakage were simulated and the effect of winter irrigation on deep leakage was analyzed. The results showed that an obvious difference existed in soil hydro-thermal dynamics between newly cultivated sandy cropland and old oasis cropland during the freezing-thawing period. In superficial layer (from 0 to 40 cm) and middle layer (from 40 to 100 cm), soil moisture content of the 2 types of soil reached the lowest level in January, and was basically recovered to the pre-freezing level after the soil thawed completely in April, and the soil water storages in these 2 layers in old oasis cropland were higher than that in newly cultivated sandy cropland. However, the change of soil water storage in deep layer (from 100 to 200 cm) was small for the 2 types of soil during the observation period. On January 18, 2017, the soil freezing depth of newly cultivated sandy cropland and old oasis cropland reached the highest value, and the highest value was 105 and 74 cm, respectively. Meanwhile, the duration of soil thawing process of old oasis cropland (26 d) was longer than that of newly cultivated sandy cropland (12 d). Nevertheless, the less thawing time for newly cultivated sandy cropland was detrimental to the soil water retention in spring. In addition, a significant difference in the deep leakage also existed between the 2 types of soil caused by winter irrigation. The deep leakage amount of newly cultivated sandy cropland was 4.1 times that of old oasis cropland when the winter irrigation amount was 160 mm. Above results suggested that the supplementary irrigation was necessary to ensure the germination of crops in spring in newly cultivated sandy cropland. Moreover, the "water preservation" effect in old oasis cropland was better than newly cultivated sandy cropland. As for different soil types, "water preservation" effect and lower deep leakage loss should be considered to ensure the optimal utilization of irrigation water resources in the desert oasis farmland management.