延安顾屯流域沟道新造耕地土壤水分动态对不同降雨年型降雨的响应

Response of soil water dynamics to rainfall variability in the newly-created valley farmland: a case study from the Gutun watershed, Yan’an City

关键词:  黄土高原  治沟造地  土壤水分  降雨  时间序列分析

Key words:  Loess Plateau  gully land consolidation  soil water  rainfall  time series analysis

摘要:

治沟造地工程是黄土高原实施的一项重大土地整治工程。为明晰治沟造地工程建设中新造耕地的土壤水分动态变化规律，基于2019—2020年延安顾屯流域沟道新造耕地土壤含水量的连续监测数据，结合时间序列分析方法，分析干旱年（2019年）与平水年（2020年）0—300 cm深土壤水分动态对降雨的响应特征。结果表明：不同降雨年型同一土层土壤含水量差异显著（P＜0.05）；平水年土壤水分活跃层、次活跃层和稳定层深度均大于干旱年；浅层土壤水分对独立降雨事件的响应在干旱年更灵敏，且干旱年0—20 cm土层的最小有效降雨量小于平水年；土壤水分对持续性降雨事件响应的滞后时间随土层深度增大而增大，平水年持续性降雨事件对土壤水分的补给速率、补给深度和补给量均大于干旱年。

Abstract:

Background, aim, and scope Soil water plays a crucial role in sustaining agricultural development in arid and semi-arid regions. Understanding the response of soil water to rainfall variability is essential for managing water resources effectively, particularly in water-limited agricultural areas facing global climate change. The Loess Plateau’s hilly and gully region is vital for rainfed agriculture in China. Recently, the ‘Gully Land Consolidation’ (GLC) project aimed to enhance watershed topography, creating high-quality farmland suitable for modern agriculture. While it is suggested that the GLC project may impact the seasonal dynamics of soil water, the relationship between soil water and rainfall variability in different rainfall years remains unclear for the newly-created valley farmland. This study aims to analyze the seasonal and interannual variations in soil water (at depths of 0—300 cm) during a drought year (2019) and a normal year (2020), and quantify the response characteristics of soil water dynamics to rainfall variability in the GLC-created farmland of the Gutun watershed, Yan’an City. Materials and methods Soil volume water content (SWC) was continuously monitored at 10 depths (i.e., 5, 10, 20, 40, 60, 100, 150, 200, 250 and 300 cm) with a 30-min frequency during 2019—2020. Rainfall and evapotranspiration data were synchronously collected using an automatic meteorological station equipped with an eddy-covariance system. Time series analysis and classical statistical methods were employed to assess the influences of rainfall variability on soil water dynamics, focusing on (1) SWC response at the 0—40 cm soil layer to independent rainfall events of different intensities, and (2) SWC response of SWC at the 0—300 cm depth to continuous rainfall events. Results SWC varied significantly between the drought year and the normal year, especially at depths of 0—100 cm. Depths of active, sub-active, and stable soil water layers were greater in the normal year. Soil water responses to independent rainfall events in the shallow layers (0—40 cm) were more sensitive during the drought year. The minimum effective rainfall for the 0—20 cm soil layers was also smaller in the drought year compared to the normal year. The lag time of SWC to continuous rainfall events increased with the depths in both the drought and normal years. The recharge rates, depths and amounts of soil water by continuous rainfall events were higher in the normal year. Discussion Spatial and temporal variations in SWC result from environmental factors such as climate, topography, soil properties, vegetation, and human activities. The GLC project significantly increased soil water storage capacity of valley farmland due to gentler terrain. Rainfall’s impact on SWC is influenced by factors including rainfall intensity, duration, soil properties, and land use types. The response of soil water to rainfall varies in different rainfall years, closely related with rainfall pattern and initial soil water content. Light rainfall effectively replenishes soil water in the shallow layer (0—40 cm) but has a more limited impact on deeper soil water. The replenishment of deep soil water primarily depends on rainfall intensity and initial soil water conditions. Specifically, the depth of soil replenished by rainfall infiltration increases with both rainfall amount and pre-rainfall SWC. During the study period, the maximum infiltration depth was only about 60 cm in the drought year, compared to up to 300 cm in the normal year. Conclusions Rainfall significantly affects soil water dynamics in GLC-created valley farmland in the Gutun watershed with greater recharge rates and depths in normal years compared to drought years. Recommendations and perspectives Understanding soil water dynamic and its response to rainfall variability in GLC-created valley farmland could guide the GLC project implementation and soil water utilization in the Loess Plateau’s hilly and gully region.