Vegetation and Roughness Controls on Field Scale Soil Moisture Variability
Downscaling of satellite-based passive microwave soil moisture products such as those to be derived from the Soil Moisture and Ocean Salinity (SMOS) mission requires enhanced understanding of controls of field scale soil moisture variability. A RADARSAT 2 field validation campaign was conducted in July 2008 to measure soil conditions, crop parameters and surface roughness over a six day period, at a network of 10 agricultural sites in Saskatchewan (N 50° - N 51°; W 105° - W 106°). Four crop types are analysed: pulse crops, cereals, oilseeds, and fallow fields, with a sample area of 2.1 km2 per site. From this data set we evaluate the impact of vegetation type and surface roughness on field scale soil moisture variability using parametric and non-parametric statistical approaches. Our results demonstrate the importance of both field scale roughness and vegetation type on field scale variability. Of significance, field scale roughness can be measured from satellite platforms such as RADARSAT-2 and vegetation type is available from optical sensors.
Effect of spatial soil hydraulic properties on large scale evapotranspiration
This study investigates the effects of different under shrub and interspace soil hydraulic property characteristics on hydrological processes, typical of desert environments. Spatial structure of soils and hydraulic properties of interspersed bare soil and shrub is important in understanding large scale hydrological processes important to the environments and ecosystems. The impact on the large scale evapotranspiration of spatial variability of shrub coverage is investigated with different ranges of shrub coverage and correlation lengths of the shrub distributions. Two fields for the soil hydraulic parameters are generated based on distinct statistics for the under shrub and interspace soil hydraulic properties. These two fields are integrated into complete spatial distribution for the soil hydraulic parameters of large scale field. The hydrological process is then simulated at the local scale and then aggregated to represent large scale evaporation and transpiration. Results demonstrate that spatial variability of soil hydraulic properties affects water partition at small scale. The ratio of shrub transpiration and total evapotranspiration at large scale increases between precipitation events and slightly drops immediately after rain. It indicates evaporation in interspace and shrub transpiration have different temporal scales.
Effective Soil Hydraulic Properties for Transient Evaporation in Heterogeneous Soils
This study investigates the use of effective soil hydraulic properties applicable to large scale evaporation problems in a landscape with horizontally heterogeneous soil hydraulic properties. The main objectives are to investigate: 1) which effective soil hydraulic property schemes are suitable to represent average behavior of large scale evaporation processes, 2) how the effective hydraulic parameters are sensitive to the process time frame. The heterogeneous landscape is represented by a series of vertically homogeneous stream tubes or parallel columns. Large scale average evaporation behavior in the heterogeneous soils is quantified through multiple simulations of local scale evaporation and aggregation. The effective hydraulic parameters are then calculated that minimize the difference between average cumulative evaporation and cumulative evaporation based on a single set of effective parameters. The effects of variances and correlations of local scale hydraulic parameters, and other conditions on the effective hydraulic properties are examined and discussed.
The Spatial and Temporal Variability of Water Content in an Organic Soil in Dartmoor National Park, UK and its Relation to Microtopography and Organic Soil Horizon Depth.
The water content of organic and mineral soils is an important parameter which links energy and mass balances at the earth's surface and as such is essential to understanding the spatial and temporal organization of many biological, biogeochemical, and hydrological processes. The characterization of surface water content in space and time is also important for the continued development of regional-scale and global circulation climate models and has large implications for agriculture and land-use planning. A field study was performed in Dartmoor National Park, Devon, UK in August 2008 for the purpose of exploring the predictive power of terrain indices on wetness patterns in an organic soil. Point samples were taken over the course of three days on two hill slopes of varying aspect in order to assess the influence of incident solar radiation on water storage. Additionally, the depth of the organic layer was estimated for each sample location and topographic information collected for the creation of a digital elevation model. A weak correlation between peat water content and organic soil layer depth was demonstrated and found to be strongest in shallow soils. Microtopography was found to influence the variability of soil moisture over the sampled area with surface roughness (measured by using residual elevation from the mean transect slope). Based on repeated observations over the sampling grids temporal persistence of water content patterns is evident and can be linked to terrain indices and depth of the organic layer.
Modelling IP3 Watersheds: Determining Retained Soil Moisture Using Both Field Capacity and Topography
An important characteristic of a distributed hydrologic model is its representation of the sub-grade processes. It has become common practice to organize the calculations of the energy and water balances on a landcover basis and to treat the watershed as a contiguous collection of categorized landscape elements. These supply runoff to micro drainage systems that in turn deliver water to major drainage systems. The storage/runoff characteristics of these elements have a direct influence on land surface processes including evaporation, infiltration, surface runoff, interflow, and recharge. Part of the IP3 modelling effort is to identify the key processes occurring in the elements for varying landscape types and how to best incorporate the conceptualization of each process into mathematical models. In many cases, soil drainage processes may be represented using a series of sloping soil layers that are subject to infiltration, percolation, and downslope interflow. To be successful, such an approach requires a means to calculate the distribution of retained water in the sloping soil horizons as a function of time. Traditionally soil moisture is represented by conceptually sound but somewhat arbitrary functions. For example, retained water was first unconstrained in WATDRAIN, the soil moisture module in MESH. Then, in WATDRAIN2, field capacity was used as a limit, defined as the water remaining in the soil when suction is at one third atmosphere. In both cases the model had difficulty in dry conditions. A new approach is proposed for near surface flow that includes an approximate solution to Richard's Equation for a sloped aquifer for both saturated and unsaturated conditions, as well as a definition of field capacity based on soil properties and topography. The results for field capacity are compared with the original data sets used to determine the one third atmosphere definition. The impact of this revised approach on simulation results is demonstrated.