Abstract
Ground-based measurement of the spatial distribution of soil moisture can be difficult because sampling is essentially made at a point and the choice of both sample depth and sample spacing affects the interpretation of the measurements. Hydrological interest has generally been in soil moisture of the root zone. Microwave Remote Sensing methods are now available that allow the interpretation of spatial distributions of soil moisture, however, their signals respond to moisture in the upper few centimetres of soil. These instruments are still being developed, but one of the questions surrounding their application is how to interpret the surface moisture in a hydrological context. In this study we compare measurements of soil moisture in 0–30 cm of soil with those in 0–6 cm to examine how representative this surface measure is with regard to the root zone. Detailed spatial measurements of soil moisture were conducted at three pasture sites in the 50 km 2 Mahurangi River catchment of northern New Zealand as part of a comprehensive hydrology project; MARVEX (MAhurangi River Variability EXperiment). In three field sites, on each of three occasions, field measurements were made using both 30 and 6 cm dielectric-based instruments. Spatial grids of several hundred moisture measurements were collected over 0–30 cm and compared with those collected simultaneously over 0–6 cm. Results indicate that temporal and spatial issues interfere with correlation of the two sets of series. Rapid wetting of 0–6 cm compared with 0–30 cm is seen following storm activity. Some evidence of the decoupling of moisture content response is also evident when sites are measured on days following a storm. Rapid, but not unrealistic, response to intense rainfall was also observed. Implications are that detailed and accurate knowledge of local soil conditions and a sound model of soil water redistribution are required before surface soil moisture measurements can be used to infer root zone behaviour. Such knowledge was not available in this study, from either published data or field observation. In this study, without suitable a priori knowledge, soil property information was found via calibration.
Highlights
Soil moisture is of critical importance to hydrological processes at a variety of scales (Bardossy and Lehmann 1998; Western et al, 2002)
Measurements of the spatial distribution of soil moisture are presented for Satellite Station (Fig. 4), Clayden’s (Fig. 5) and Carran’s (Fig. 6)
We estimated the degree to which R2 is affected by these errors by assuming that the total variance in each measurement was reduced by the random measurement error, and that the covariance was unaffected
Summary
Soil moisture is of critical importance to hydrological processes at a variety of scales (Bardossy and Lehmann 1998; Western et al, 2002). Previous studies have highlighted spatial organisation at 0 – 30 cm depth at some times of the year and a more random pattern at other times (Grayson et al, 1997; Western and Grayson 1998). Such variation has implications for runoff generation and the scale characteristics applied to spatial averaging of processes such as evapotranspiration. The only method for directly determining soil moisture is gravimetric sampling, this is time consuming to undertake and difficult to use for the estimation of spatial patterns. One of the most reliable and portable methods of measuring for soil moisture content is time-domain reflectometry (TDR) (Topp et al, 1980; Roth et al, 1992; Western et al, 2001b)
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