SOIL MOISTURE VARIABILITY: IMPLICATIONS FOR THE HYDROLOGY, EROSION AND MANAGEMENT OF GULLIED CATCHMENTS IN CENTRAL SPAIN

In semi-arid environments, the combination of a non-uniform distribution of vegetation, an often highly irregular terrain and complex geological, pedological and management histories have frequently given rise to considerable spatial variability in the physical and hydrological properties of soils. Heterogeneity within the soil's physical and hydrological properties can result in pronounced differences in infiltration and soil moisture. The hydrological response of semi-arid landscapes to rainfall events may therefore be spatially non-uniform. Quantifying the spatial pattern of hydrological response is important for identifying those areas within the landscape which arc vulnerable to runoff and erosion. Since soil moisture is considered to be a key factor in determining hydrological response and its spatial distribution is a function of the soil's physical and hydrological properties, the spatial and temporal measurement of soil moisture may be used to identify contrasting areas of hydrological response. In a badlands environment located approximately 70 km north of Madrid, central Spain, an experiment was established to describe the temporal and spatial variability in soil moisture at three scales, with the primary aim of furthering the understanding of the hydrological and geomorphological processes operating in semi-arid landscapes. At each measurement scale, the macroscale (25m sampling interval), the mesoscale (gully catchments, 5m sampling interval) and the microscale (1 m sampling interval), two distinct groups of soil moisture conditions emerged related to dry and wet weather conditions. At each measurement scale the maximum variability in soil moisture is similar (>20% volumetric content difference between immediately adjacent sampling points). At the meso and microscale the spatial pattern of soil moisture could be described as a mosaic pattern which during the dry period was more fragmented and variable than during the wet period. The spatial pattern of soil moisture during wet conditions is more uniform due to the development of extensive wet areas within the catchments. During these conditions the range of spatial correlation in soil moisture may double (to greater than 30m) compared to dry conditions, indicating an increase in the spatial continuity of soil moisture. The spatial variability in soil moisture therefore displays a temporal dependency; the mosaic soil moisture pattern is more fragmented and spatially discontinuous during dry than wet conditions. A striking characteristic of the study area is the near horizontal interbedding of sediment horizons which may strongly contrast in their textural composition over relatively short distances. This variability in soil texture and the associated changes in pore size characteristics, were the principal controlling factors in determining the spatial patterns of soil moisture and overrides the known influence of vegetation and topography on soil moisture. During dry conditions the non-uniform uptake of soil moisture by vegetation may partly explain the greater variability in soil moisture observed during this period. The mosaic patterns of soil moisture represent areas of contrasting hydrological response. During dry periods when the mosaic pattern is more fragmented, source areas of overland flow are spatially isolated and surrounded by 'sink' areas capable of re-absorbing runoff and sediment deposition. Hydrological pathways are therefore discontinuous resulting in minimal runoff reaching the catchments channels. Since soil moisture values during this period are below saturation, any runoff which does occur is generated as infiltration excess overland flow. In semi-arid areas spatial variability in soil properties or vegetation patterns may therefore be beneficial for runoff and erosion control by creating a self-regulating system in which runoff producing areas are surrounded by buffer zones capable of re-absorbing the runoff. During wet periods extensive areas of the catchments may be saturated. source areas are no longer spatially isolated and continuous hydrological pathways may develop rapidly during this period. During the wet period when conditions arc above a critical saturation threshold value widespread runoff will occur regardless of the spatial variability in the soil's physical and hydrological properties. The creation of a mosaic pattern in which buffer zones are adjacent to potential runoff producing areas, as identified from spatial soil moisture patterns, may provide the most effective management strategy in runoff and erosion control for degraded semi-arid environments. The creation of a mosaic pattern is most applicable at the watershed scale allowing several land uses, including those which are potentially degrading, to co-exist. Increasing the critical threshold value above which widespread runoff occurs should also.be included as part of this management strategy

Controls on the temporal and spatial variability of soil moisture in a mountainous landscape: the signature of snow and complex terrain

The controls on the spatial distribution of soil moisture include static and dynamic variables. The superposition of static and dynamic controls can lead to different soil moisture patterns for a given catchment during wetting, draining, and drying periods. These relationships can be further complicated in snow-dominated mountain regions where soil water input by precipitation is largely dictated by the spatial variability of snow accumulation and melt. In this study, we assess controls on spatial and temporal soil moisture variability in a small (0.02 km<sup>2</sup>), snow-dominated, semi-arid catchment by evaluating spatial correlations between soil moisture and site characteristics through different hydrologic seasons. We assess the relative importance of snow with respect to other catchment properties on the spatial variability of soil moisture and track the temporal persistence of those controls. Spatial distribution of snow, distance from divide, soil texture, and soil depth exerted significant control on the spatial variability of moisture content throughout most of the hydrologic year. These relationships were strongest during the wettest period and degraded during the dry period. As the catchment cycled through wet and dry periods, the relative spatial variability of soil moisture tended to remain unchanged. We suggest that the static properties in complex terrain (slope, aspect, soils) impose first order controls on the spatial variability of snow and resulting soil moisture patterns, and that the interaction of dynamic (timing of water input) and static influences propagate that relative constant spatial variability through most of the hydrologic year. The results demonstrate that snow exerts significant influence on how water is retained within mid-elevation semi-arid catchments and suggest that reductions in annual snowpacks associated with changing climate regimes may strongly influence spatial and temporal soil moisture patterns and catchment physical and biological processes

Spatiotemporal analyses of soil moisture from point to footprint scale in two different hydroclimatic regions

This paper presents time stability analyses of soil moisture at different spatial measurement support scales (point scale and airborne remote sensing (RS) footprint scale 800 m × 800 m) in two different hydroclimatic regions. The data used in the analyses consist of in situ and passive microwave remotely sensed soil moisture data from the Southern Great Plains Hydrology Experiments 1997 and 1999 (SGP97 and SGP99) conducted in the Little Washita (LW) watershed, Oklahoma, and the Soil Moisture Experiments 2002 and 2005 (SMEX02 and SMEX05) in the Walnut Creek (WC) watershed, Iowa. Results show that in both the regions soil properties (i.e., percent silt, percent sand, and soil texture) and topography (elevation and slope) are significant physical controls jointly affecting the spatiotemporal evolution and time stability of soil moisture at both point and footprint scales. In Iowa, using point‐scale soil moisture measurements, the WC11 field was found to be more time stable (TS) than the WC12 field. The common TS points using data across the 3 year period (2002–2005) were mostly located at moderate to high elevations in both the fields. Furthermore, the soil texture at these locations consists of either loam or clay loam soil. Drainage features and cropping practices also affected the field‐scale soil moisture variability in the WC fields. In Oklahoma, the field having a flat topography (LW21) showed the worst TS features compared to the fields having gently rolling topography (LW03 and LW13). The LW13 field (silt loam) exhibited better time stability than the LW03 field (sandy loam) and the LW21 field (silt loam). At the RS footprint scale, in Iowa, the analysis of variance (ANOVA) tests show that the percent clay and percent sand are better able to discern the TS features of the footprints compared to the soil texture. The best soil indicator of soil moisture time stability is the loam soil texture. Furthermore, the hilltops (slope ∼0%–0.45%) exhibited the best TS characteristics in Iowa. On the other hand, in Oklahoma, ANOVA results show that the footprints with sandy loam and loam soil texture are better indicators of the time stability phenomena. In terms of the hillslope position, footprints with mild slope (0.93%–1.85%) are the best indicators of TS footprints. Also, at both point and footprint scales in both the regions, land use–land cover type does not influence soil moisture time stability

Temporal stability of soil moisture and radar backscatter observed by the advanced Synthetic Aperture Radar (ASAR)

The high spatio-temporal variability of soil moisture is the result of atmospheric forcing and redistribution processes related to terrain, soil, and vegetation characteristics. Despite this high variability, many field studies have shown that in the temporal domain soil moisture measured at specific locations is correlated to the mean soil moisture content over an area. Since the measurements taken by Synthetic Aperture Radar (SAR) instruments are very sensitive to soil moisture it is hypothesized that the temporally stable soil moisture patterns are reflected in the radar backscatter measurements. To verify this hypothesis 73 Wide Swath (WS) images have been acquired by the ENVISAT Advanced Synthetic Aperture Radar (ASAR) over the REMEDHUS soil moisture network located in the Duero basin, Spain. It is found that a time-invariant linear relationship is well suited for relating local scale (pixel) and regional scale (50 km) backscatter. The observed linear model coefficients can be estimated by considering the scattering properties of the terrain and vegetation and the soil moisture scaling properties. For both linear model coefficients, the relative error between observed and modelled values is less than 5 % and the coefficient of determination (R-2) is 86 %. The results are of relevance for interpreting and downscaling coarse resolution soil moisture data retrieved from active (METOP ASCAT) and passive (SMOS, AMSR-E) instruments

Assessing temporal stability for coarse scale satellite moisture validation in the Maqu area, Tibet

This study evaluates if the temporal stability concept is applicable to a time series of satellite soil moisture images so to extend the common procedure of satellite image validation. The area of study is the Maqu area, which is located in the northeastern part of the Tibetan plateau. The network serves validation purposes of coarse scale (25–50 km) satellite soil moisture products and comprises 20 stations with probes installed at depths of 5, 10, 20, 40, 80 cm. The study period is 2009. The temporal stability concept is applied to all five depths of the soil moisture measuring network and to a time series of satellite-based moisture products from the Advance Microwave Scanning Radiometer (AMSR-E). The in-situ network is also assessed by Pearsons’s correlation analysis. Assessments by the temporal stability concept proved to be useful and results suggest that probe measurements at 10 cm depth best match to the satellite observations. The Mean Relative Difference plot for satellite pixels shows that a RMSM pixel can be identified but in our case this pixel does not overlay any in-situ station. Also, the RMSM pixel does not overlay any of the Representative Mean Soil Moisture (RMSM) stations of the five probe depths. Pearson’s correlation analysis on in-situ measurements suggests that moisture patterns over time are more persistent than over space. Since this study presents first results on the application of the temporal stability concept to a series of satellite images, we recommend further tests to become more conclusive on effectiveness to broaden the procedure of satellite validation

Ecohydrologic Impacts of Rangeland Fire on Runoff and Erosion: A Literature Synthesis

Fire can dramatically influence rangeland hydrology and erosion by altering ecohydrologic relationships. This synthesis presents an ecohydrologic perspective on the effects of fire on rangeland runoff and erosion through a review of scientific literature spanning many decades. The objectives are: (1) to introduce rangeland hydrology and erosion concepts necessary for understanding hydrologic impacts of fire; (2) to describe how climate, vegetation, and soils affect rangeland hydrology and erosion; and (3) to use examples from literature to illustrate how fire interacts with key ecohydrologic relationships. The synthesis is intended to provide a useful reference and conceptual framework for understanding and evaluating impacts of fire on rangeland runoff and erosion