Estimation of Average Soil Water Content at the Surface Soil Layer in a Headwater Catchment

• Main Authors: Sheng-Lun Li, 李勝倫
• Other Authors: 梁偉立
• Format: Others
• Language: zh-TW
• Published: 2015
• Online Access: http://ndltd.ncl.edu.tw/handle/43277478955078294025

碩士 === 國立臺灣大學 === 森林環境暨資源學研究所 === 103 === Surface moisture (θ) in a forested headwater catchment would vary greatly in space and time due to the influences of topography, vegetation, and soil properties. In addition to the monitoring network with highly spatial and temporal resolutions, developing an effective method to measure the average surface moisture with few samples would be efficient to understand the hydrological responses and interactions at the atmosphere-soil interface in a forested headwater catchment. In this study, we tried to estimate average surface moisture with a small sample size and to find the locations with representativeness. We measured θ at the depths of 0-12 cm at 459 measurement points using portable TDR (time domain reflectometry) sensors. Twelve surveys were conducted within 1 year in a natural forested headwater catchment. Temporal stability of each measurement point was analyzed by the relative difference approach. The mean soil moistures determined by temporal stability and systematic sampling methods were compared to average surface moisture for each survey. The minimum sample size necessary to estimate average surface moisture was determined with the residual sum of squares (RSS) and r2 value of linear regression between mean soil moistures and average surface moisture. We also analyzed the relationships of the representative samples (Sr) and environmental factors (i.e. topography, vegetation, and soil properties) in space and time. The results showed that 8 samples with minimum index of temporal stability (ITS) were sufficient to estimate average surface moisture, while more than 15 systematic samples were necessary. The correlations between the spatial distributions of Sr and environmental factors were weak. Most of Sr located at the valley-head hillslope and gullies. Gullies were not necessarily good locations for Sr where the best and worst representative samples existed. The spatial distribution of Sr varied with the dry and wet conditions. When average surface moisture was lower, the spatial pattern of Sr could not be characterized by environmental factors. When average surface moisture was higher, Sr tended to locate around the center of the headwater catchment.