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Abstract: This paper analyzes the potential of the radar altimeter onboard ENVISAT for estimating surface soil moisture in the semi-arid Gourma region in Northern Mali. To this end, the relationships between observed backscattering coefficients derived from 4 retracking algorithms, namely Ocean, Ice-1, Ice-2 and Sea-Ice, and ground data, including soil type, topography, vegetation and soil moisture are investigated. The considered period is 2002–2010. Results show a strong linear relationship between the backscattering coefficients and surface soil moisture measured at six different stations along the satellite track. The best results are obtained with the Ice-1 and Ice-2 algorithms. In these cases, correlation coefficients are higher than 0.8 with RMSE smaller than 2%. Vegetation effects are found to be small due both to the nadir-looking configuration of the radar altimeter and to the low vegetation cover. Finally, the relationship between soil moisture and normalized backscattering coefficient is used to retrieve soil moisture from the altimeter data. These estimates are then compared to soil moisture estimations obtained from the METeorological Operational (METOP) Advanced SCATterometer (ASCAT). These results highlight the high capabilities of Ku-band altimeters to provide an accurate estimation of surface soil moisture in semiarid regions. [Copyright &y& Elsevier]

Abstract: Although numerous factors influence soil-water content, it is considered a key indicator of rangeland health. This paper investigates the effect of grazing on soil-water content using three treatments within the same soil association. The treatments, simulated holistic planned grazing (SHPG), rest-rotation (RESTROT), and total rest (TREST) applied stocking rates of 36, 6, and 0 animal days/hectare respectively. Soil-water content was measured continuously from 2006 to 2008 using 36 capacitance sensors. Statistical analyses revealed differences in percent volumetric-water content (%VWC) and in all treatments, the SHPG pasture had the highest %VWC. Mixed procedures models indicate strong environmental and treatment effects as explanatory variables for the observed difference in %VWC. Although results of vegetation cover analyses indicated no difference in percent shrub cover in the two production pastures (SHPG and RESTROT), percent litter cover differed in the latter years of this study. It was concluded that in addition to a variety of other factors, management decisions (grazing and rest) can have substantial influence upon soil-water content and that soil-water content can vary substantially as a result of animal impact and the duration of grazing. [Copyright &y& Elsevier]

Abstract: In the Tarim River Basin, the desert riparian forest vegetation is under high-temperature and aridity stress. However, the vegetation can grow continuously because of deep rooting that can reach groundwater, which can thus redistribute water into the upper soil profile. This paper describes patterns of hydraulic lift by Populus euphratica Oliv. and discusses its ecological effects. Our results show that the tap root sap velocity of P. euphratica Oliv. is positive during the day and night. However, a reverse sap flow was observed in the lateral roots during the night. The soil water content of the subsoil was higher than that of the topsoil at depths of 0–120cm. When the sap flow of the lateral roots was reversed at night, the soil water content clearly increased. In particular, at depths of 60–120cm, the soil water content at 4:00 was 28–38% greater than that at 16:00. The vapor pressure deficit was a factor that predominantly affected the root sap velocity, and the smaller vapor pressure deficit often facilitated a reverse sap flow in the lateral roots. Our findings demonstrate the hydraulic lift characteristics and ecological effects that occur in the desert riparian forest in extremely arid regions of middle Asia [Copyright &y& Elsevier]

Accurate prediction and characterization of the spatial distribution and temporal variation of soil water is indispensable to optimize the use and management of land within dry areas, which suffer from limited arable land and water resources. The objective of this research is to examine the effect of hillslope position and characteristics on soil properties and variation of soil moisture in an arid to semiarid region. Five transects representing toposequences in the transitional Arid to Semiarid Mediterranean region of Jordan were selected to study the effect of hillslope position on soil properties and variation of soil moisture. Four sites representing different hillslope positions were chosen within each transect from the summit down to the toeslope. Soil profile, at each position, was described and sampled for laboratory analyses. Moisture content at different depths was monitored during the rainy season. The results indicated a systematic relationship between topographic characteristics (hillslope position, slope steepness, and curvature) and the particle size distribution. Soil erodibility was found to be indirectly related to topographic position as extrapolated from the consistent relationship with clay content. Soil moisture content was related to the topographic position, the variation in clay content, and vegetation condition. The topographic position was found to govern the flow of water from upper to lower positions, resulting in higher moisture availability at lower positions. The variations of other parameters were not only related to the topographic position of particular soil, but also to the characteristics of the soil at the upper positions, and to the sequence in which that soil is positioned in the toposequence. Understanding the distribution of soil properties and their relationship along the soil-landscape continuum, which is unique to this environment, should improve the prediction of soil properties and improve plans to better manage arid land. [ABSTRACT FROM AUTHOR]

The Tropical Rainfall Measuring Mission (TRMM) carries aboard the Precipitation Radar (TRMMPR) that measures the backscatter (σ°) of the surface. σ° is sensitive to surface soil moisture and vegetation conditions. Due to sparse vegetation in arid and semi-arid regions, TRMMPR σ° primarily depends on the soil water content. In this study we relate TRMMPR σ° measurements to soil water content (ms ) in the Lower Colorado River Basin (LCRB). σ° dependence on ms is studied for different vegetation greenness values determined through Normalized Difference Vegetation Index (NDVI). A new model of σ° that couples incidence angle, ms, and NDVI is used to derive parameters and retrieve soil water content. The calibration and validation of this model are performed using simulated and measured ms data. Simulated ms is estimated using the Variable Infiltration Capacity (VIC) model and measured ms is acquired from ground measuring stations inWalnut Gulch Experimental Watershed (WGEW). σ° model is calibrated using VIC and WGEW ms data during 1998 and the calibrated model is used to derive ms during later years. The temporal trends of derived ms are consistent with VIC and WGEW ms data with a correlation coefficient (R) of 0.89 and 0.74, respectively. Derived ms is also consistent with the measured precipitation data with R=0.76. The gridded VIC data is used to calibrate the model at each grid point in LCRB and spatial maps of the model parameters are prepared. The model parameters are spatially coherent with the general regional topography in LCRB. TRMMPR σ° derived soil moisture maps during May (dry) and August (wet) 1999 are spatially similar to VIC estimates with correlation 0.67 and 0.76, respectively. This research provides new insights into Ku-band σ° dependence on soil water content in the arid regions. [ABSTRACT FROM AUTHOR]

The objective of the present study was to investigate the potential of a large aperture scintillometer (LAS) combined with a simple available energy model to estimate area-averaged latent heat flux in difficult environmental conditions. The difficulties are related to the sparseness of the vegetation, the heterogeneity of the soil characteristics, and, most importantly, the heterogeneity in terms of soil moisture induced by the “flood irrigation” method. In this context, three sites (Agdal, R3 and Saada) in the Tensift Al Haouz plain (region of Marrakech city, central Morocco) have been equipped with a LAS and eddy covariance (EC) system (local scale measurements). Agdal and R3 are a flood-irrigated olive yard and wheat field, respectively. Saada is a drip-irrigated orange orchard. Due to the irrigation method applied, the Agdal and R3 sites shifted from being almost homogeneous between two irrigations (dry conditions) and completely heterogeneous during the irrigation events (large variability of soil moisture along the site), while Saada was always heterogeneous, at least at the scintillometer footprint scale. Consequently, the comparison between the sensible heat fluxes derived from both LAS and EC showed a large scatter during the irrigation events, while a good correspondence was found in between two irrigations. It was also found that combining LAS and an estimate of the available energy (using a simple model) can provide reasonable large-scale evapotransipration estimates, which are of prime interest for irrigation management. [ABSTRACT FROM AUTHOR]