Lateral Redistribution Of Water Research Articles

FEATURES OF ECOLOGICAL GEOMORPHOMETRY AS A PROSPECTIVE FIELD OF STUDY, ITS MAIN CONCEPTS AND METHODS

Abstract. The main concepts and methods of ecological geomorphometry as a research field aimed at studying relationships between terrain morphometric characteristics and ecological factors and processes are reviewed in the paper. The progress in this research field has been conditioned by the propagation of high-resolution digital elevation models in free access and of methods of their digital analysis, namely: the calculation of derivative quantitative characteristics (attributes and indices) of terrain and the statistical models of analyzing the relationships between the latter and the ecological properties and factors (those relevant for a certain ecological subject). A peculiar feature of ecological approach to regarding terrain (its morphology) is subjectcentrism (that is, regarding it from a point of view of a certain subject). The subject of ecological relationships can be living entities (populations, species, communities) as well as a human, social entities, economy and its branches. Three main concepts of ecological geomorphometry are put forward: terrain attributes (relatively simple quantitative characteristics of terrain form that characterize its geometry and some elementary physical processes); topographic indices (quantitative surrogates for some complex physical or biophysical processes of ecological significance); morphotops (spatial units that are distinguished by terrain morphology, using criteria of ecological homogeneity relevant from a viewpoint of a certain ecological subject). Morphotops can be distinguished with different level of detail (and, as a result, with different characteristic dimensions), relative to the study aim, to the geographic features of the area, and to the available data and the methods of their analysis. While morphotops are distinguished with strictly defined quantitative morphometric parameters (terrain attributes, topographic indices), this enables using formalized methods with their advantages of reproducibility and possibility of automatizing. In our studies aimed at morphotop mapping for a small area in the hilly terrain of Davydiv range near Lviv and for a larger area in the central part of Ukrainian Carpathians, morphotops delineation was based on topographic indices that characterize insolation level (solar radiation incidence on terrain elements of different aspect and slope values), lateral redistribution of water on slopes and redistribution of solid matter by washout on slopes. Morphotops were distinguished with cluster analysis method, which allows to distinguish natural groupings of data in the attribute space. Presetting different number of clusters to be distinguished, morphotopes can be distinguished with different levels of detail, larger number of clusters corresponding to more homogenous morphotops with smaller characteristic sizes. Key words: ecological geomorphology, ecological geomorphometry, morphotops, terrain attributes, topographic indices.

Sensitivity of nitrate concentration‐discharge patterns to soil nitrate distribution and drainage properties in the vertical dimension

We propose a conceptual model that examines the ‘variable source area’ (VSA) and ‘nitrate flushing’ hypothesis in the vertical direction, and use this approach to explain nitrate concentration–discharge relationships in a semi-arid watershed. We use an eco-hydrology simulation model (RHESSys) to show that small changes in the vertical distribution of nitrate mass and their interaction with soil hydraulic conductivity can result in abrupt changes in the nitrate concentration–discharge relationship. We show that the estimated concentration–discharge relationship is sensitive to the parameters governing soil vertical nitrate distribution and soil hydraulic conductivity, at both patch scale and watershed scale, where lateral redistribution of water and nitrate is also accounted for. Given heterogeneity in nitrogen inputs, uptake processes, soil drainage and storage processes, substantial variation in parameters that describe rate of changes in vertical distribution of soil nitrate and hydraulic properties is likely both within and between watersheds. Thus, we argue that vertical ‘variable source area’ processes may be as important as lateral VSA in determining concentration discharge relationships.

Effects of soil and vegetation development on surface hydrological properties of moraines in the Swiss Alps

The near-surface saturated hydraulic conductivity (Ksat) is an important hydrological characteristic because it determines surface infiltration rates and the vertical and lateral redistribution of water in the soil. However, there is comparatively little knowledge about the changes in Ksat during landscape development and how the co-evolution of biological, pedological and hydrological characteristics affect the movement of water through the soil. On the one hand, increasing vegetation cover is expected to increase macroporosity and thus Ksat. On the other hand, clay formation is expected to decrease Ksat. To investigate how hillslope aging affects Ksat, we used a space-for-time approach and conducted comprehensive measurements of vegetation, soil and topography on a chronosequence of moraines in a proglacial area of the Swiss Alps. On four moraines, ranging from about ten thousand to ~30 years in age, we measured for three plots the near-surface soil characteristics. Surface and near-surface Ksat were high and decreased with depth on all moraines. Surface Ksat was highest on the youngest moraine (median: 4320 mm hr−1) and lowest (540 mm hr−1) on the oldest moraine. Ksat was significantly positive correlated with soil texture and the gravel content in the surface soil layer. The correlation analyses and Structural Equation Model suggested that the larger fraction of small particles for the older moraines had a bigger effect on Ksat than the denser root network. Even though the variability in measured Ksat-values within the moraines was high and water movement is thus likely very heterogeneous, the measured Ksat values suggest that infiltration-excess overland flow is very unlikely on these hillslopes but (lateral) near surface flow likely increases with the age of the hillslope. This information is important for understanding differences in runoff generation mechanisms in alpine areas with moraines of different ages, as well as landscape evolution models.

Efficient estimation of effective hydraulic properties of stratal undulating surface layer using time-lapse multi-channel GPR

Abstract. Multi-scale soil architectures in shallow subsurface are widespread in natural and anthropogenic depositional environments, and acquisition of the surface stratal structure and hydrological properties are essential in quantifying water cycling. Geophysical methods like ground-penetrating radar (GPR) can provide quantitative information like soil architecture and spatiotemporal soil water content distribution for the shallow layer. Concerning the informative multi-dimensional water flow in the surface layer with an undulating bottom at the plot scale, this study assesses the feasibility of efficiently estimating soil hydraulic properties using a few time-lapse multi-channel GPR observations, namely soil water storage and layer thickness of the surface layer, at reclamation land near an old river channel. We show that effective hydraulic properties of the surface layer can be obtained with a small number of time-lapse GPR measurements during a rainfall event. Additionally, we analyze the effect of some key factors controlling the informative lateral water redistribution on the results of the proposed approach using synthetic simulations.

Investigating lake-area dynamics across a permafrost-thaw spectrum using airborne electromagnetic surveys and remote sensing time-series data in Yukon Flats, Alaska

Lakes in boreal lowlands cycle carbon and supply an important source of freshwater for wildlife and migratory waterfowl. The abundance and distribution of these lakes are supported, in part, by permafrost distribution, which is subject to change. Relationships between permafrost thaw and lake dynamics remain poorly known in most boreal regions. Here, new airborne electromagnetic (AEM) data collected during June 2010 and February 2016 were used to constrain deep permafrost distribution. AEM data were coupled with Landsat-derived lake surface-area data from 1979 through 2011 to inform temporal lake behavior changes in the 35 500- km2 Yukon Flats ecoregion of Alaska. Together, over 1500 km of AEM data, and roughly 30 years of Landsat data were used to explore processes that drive lake dynamics across a variety of permafrost thaw states not possible in studies conducted with satellite imagery or field measurements alone. Clustered time-series data identified lakes with similar temporal dynamics. Clusters possessed similarities in lake permanence (i.e. ephemeral versus perennial), subsurface permafrost distribution, and proximity to rivers and streams. Of the clustered lakes, ∼66% are inferred to have at least intermittent connectivity with other surface-water features, ∼19% are inferred to have shallow subsurface connectivity to other surface water features that served as a low-pass filter for hydroclimatic fluctuations, and ∼15% appear to be isolated by surrounding permafrost (i.e. no connectivity). Integrated analysis of AEM and Landsat data reveals a progression from relatively synchronous lake dynamics among disconnected lakes in the most spatially continuous, thick permafrost to quite high spatiotemporal heterogeneity in lake behavior among variably-connected lakes in regions with notably less continuous permafrost. Variability can be explained by the preferential development of thawed permeable gravel pathways for lateral water redistribution in this area. The general spatial progression in permafrost thaw state and lake area behavior may be extended to the temporal dimension. However, extensive permafrost thaw, beyond what is currently observed, is expected to promote ubiquitous subsurface connectivity, eventually evolving to a state of increased lake synchronicity.

Accounting for landscape heterogeneity improves spatial predictions of tree vulnerability to drought.

As climate change continues, forest vulnerability to droughts and heatwaves is increasing, but vulnerability varies regionally and locally through landscape position. Also, most models used in forecasting forest responses to heat and drought do not incorporate relevant spatial processes. In order to improve spatial predictions of tree vulnerability, we employed a nonlinear stochastic model of soil moisture dynamics accounting for landscape differences in aspect, topography and soils. Across a watershed in central Texas we modeled dynamic water stress for a dominant tree species, Juniperus ashei, and projected future dynamic water stress through the 21st century. Modeled dynamic water stress tracked spatial patterns of remotely sensed drought-induced canopy loss. Accuracy in predicting drought-impacted stands increased from 60%, accounting for spatially variable soil conditions, to 72% when also including lateral redistribution of water and radiation/temperature effects attributable to aspect. Our analysis also suggests that dynamic water stress will increase through the 21st century, with trees persisting at only selected microsites. Favorable microsites/refugia may exist across a landscape where trees can persist; however, if future droughts are too severe, the buffering capacity of an heterogeneous landscape could be overwhelmed. Incorporating spatial data will improve projections of future tree water stress and identification of potential resilient refugia.

A Budyko framework for estimating how spatial heterogeneity and lateral moisture redistribution affect average evapotranspiration rates as seen from the atmosphere

Abstract. Most Earth system models are based on grid-averaged soil columns that do not communicate with one another, and that average over considerable sub-grid heterogeneity in land surface properties, precipitation (P), and potential evapotranspiration (PET). These models also typically ignore topographically driven lateral redistribution of water (either as groundwater or surface flows), both within and between model grid cells. Here, we present a first attempt to quantify the effects of spatial heterogeneity and lateral redistribution on grid-cell-averaged evapotranspiration (ET) as seen from the atmosphere over heterogeneous landscapes. Our approach uses Budyko curves, as a simple model of ET as a function of atmospheric forcing by P and PET. From these Budyko curves, we derive a simple sub-grid closure relation that quantifies how spatial heterogeneity affects average ET as seen from the atmosphere. We show that averaging over sub-grid heterogeneity in P and PET, as typical Earth system models do, leads to overestimations of average ET. For a sample high-relief grid cell in the Himalayas, this overestimation bias is shown to be roughly 12 %; for adjacent lower-relief grid cells, it is substantially smaller. We use a similar approach to derive sub-grid closure relations that quantify how lateral redistribution of water could alter average ET as seen from the atmosphere. We derive expressions for the maximum possible effect of lateral redistribution on average ET, and the amount of lateral redistribution required to achieve this effect, using only estimates of P and PET in possible source and recipient locations as inputs. We show that where the aridity index P/PET increases with altitude, gravitationally driven lateral redistribution will increase average ET (and models that overlook lateral redistribution will underestimate average ET). Conversely, where the aridity index P/PET decreases with altitude, gravitationally driven lateral redistribution will decrease average ET. The effects of both sub-grid heterogeneity and lateral redistribution will be most pronounced where P is inversely correlated with PET across the landscape. Our analysis provides first-order estimates of the magnitudes of these sub-grid effects, as a guide for more detailed modeling and analysis.

Temporal Dynamics and Stability of Spatial Soil Matric Potential in Two Land Use Systems

Core Ideas The spatial variance of ψm at the 10‐cm soil depth decreased with median ψm. Weekly precipitation and ψm had common temporal periodicities. The response of ψm to precipitation was faster and more common in cropland. Wavelet analysis revealed two different behaviors of water infiltration. The ψm spatial pattern was more related to elevation than soil texture in grassland. The spatial variability of the soil water status and its temporal behavior are essential for hydrological modeling and agricultural management. Changes in atmospheric conditions such as precipitation usually cause large temporal dynamics in the soil water status, whereas inherent soil properties such as soil texture and topography maintain their spatial pattern with time. Our objective was to identify the relevant spatial and temporal processes underlying soil water behavior. Along a 48‐ by 3‐m transect evenly distributed across two land use systems, i.e., cropland and grassland, the soil matric potential (ψm) was measured on a weekly basis from May to October 2013 at 1‐m intervals and at depths of 10, 30, 50, 70, 90, and 110 cm. The spatial variance of ψm at the 10‐cm depth decreased with weekly precipitation and the spatial median of ψm in both land use systems, owing to the enhancement of lateral water redistribution by elevated soil wetness. Temporal dynamics in ψm and its spatial variance diminished with soil depth. In contrast, the temporal stability of ψm generally increased with soil depth because of the decreasing impact exerted by precipitation. According to Spearman's rank correlation coefficients, the relative distribution of ψm in space was more strongly correlated with relative elevation than with soil texture in the grassland. These results have direct implications for irrigation management.

The spatial and temporal evolution of contributing areas

AbstractPredicting runoff source areas and how they change through time is a challenge in hydrology. Topographically induced lateral water redistribution and water removal through evapotranspiration lead to spatially and temporally variable patterns of watershed water storage. These dynamic storage patterns combined with threshold mediation of saturated subsurface throughflow lead to runoff source areas that are dynamic through time. To investigate these processes and their manifestation in watershed runoff, we developed and applied a parsimonious but spatially distributed model (WECOH—Watershed ECOHydrology). Evapotranspiration was measured via an eddy‐covariance tower located within the catchment and disaggregated as a function of vegetation structure. This modeling approach reproduced the stream hydrograph well and was internally consistent with observed watershed runoff patterns and behavior. We further examined the spatial patterns of water storage and their evolution through time by building on past research focused on landscape hydrologic connectivity. The percentage of landscape area connected to the stream network ranged from less than 1% during the fall and winter base flow period to 71% during snowmelt. Over the course of the 2 year study period, 90% of the watershed areas were connected to the stream network for at least 1 day, leaving 10% of area that never became connected. Runoff source areas during the event shifted from riparian dominated runoff to areas at greater distances from the stream network when hillslopes became connected. Our modeling approach elucidates and enables quantification and prediction of watershed active areas and those active areas connected to the stream network through time.

Vegetation and topographic influences on the connectivity of shallow groundwater between hillslopes and streams

ABSTRACTLittle is known about the combined effects of vegetation and topography on hillslope water table dynamics. In forested headwater catchments, complex terrain and vegetation intersect to impose large spatial and temporal variability in the vertical and lateral redistribution of water from hillslopes to streams. Here, we demonstrate, using empirical data from the Northern Rocky Mountains, that vegetation interacts with landscape topography to influence hillslope–riparian–stream hydrologic connectivity. We compared a measured relationship between hillslope contributing area and hydrologic connectivity during the growing season to LiDAR‐derived vegetation characteristics and found that two behavioural regimes emerged. Among some hillslopes, hydrologic connectivity decreased as vegetation density increased, demonstrating that growing season hydrologic connectivity is subject to the balance between evapotranspiration and lateral redistribution of soil water. Among other hillslopes, hydrologic connectivity increased as vegetation density increased. For the latter set of hillslopes, hydrologic connectivity cannot be explained by topography and vegetation alone. When we compared joint distributions of vegetation density and modelled solar irradiance between the two regimes as another indicator of evapotranspiration, we found that conditions were generally more favourable for higher transpiration on hillslopes where hydrologic connectivity decreased as vegetation density increased than on hillslopes where the opposite behaviour was observed. Our results demonstrate not only the importance of vegetation heterogeneity for hillslope–riparian–stream connectivity but also the importance of other spatially distributed variables such as energy availability when considering the influence of topography on hydrological processes. Copyright © 2013 John Wiley & Sons, Ltd.

Effects of vegetation heterogeneity and surface topography on spatial scaling of net primary productivity

Abstract. Due to the heterogeneous nature of the land surface, spatial scaling is an inevitable issue in the development of land models coupled with low-resolution Earth system models (ESMs) for predicting land-atmosphere interactions and carbon-climate feedbacks. In this study, a simple spatial scaling algorithm is developed to correct errors in net primary productivity (NPP) estimates made at a coarse spatial resolution based on sub-pixel information of vegetation heterogeneity and surface topography. An eco-hydrological model BEPS-TerrainLab, which considers both vegetation and topographical effects on the vertical and lateral water flows and the carbon cycle, is used to simulate NPP at 30 m and 1 km resolutions for a 5700 km2 watershed with an elevation range from 518 m to 3767 m in the Qinling Mountain, Shanxi Province, China. Assuming that the NPP simulated at 30 m resolution represents the reality and that at 1 km resolution is subject to errors due to sub-pixel heterogeneity, a spatial scaling index (SSI) is developed to correct the coarse resolution NPP values pixel by pixel. The agreement between the NPP values at these two resolutions is improved considerably from R2 = 0.782 to R2 = 0.884 after the correction. The mean bias error (MBE) in NPP modelled at the 1 km resolution is reduced from 14.8 g C m−2 yr−1 to 4.8 g C m−2 yr−1 in comparison with NPP modelled at 30 m resolution, where the mean NPP is 668 g C m−2 yr−1. The range of spatial variations of NPP at 30 m resolution is larger than that at 1 km resolution. Land cover fraction is the most important vegetation factor to be considered in NPP spatial scaling, and slope is the most important topographical factor for NPP spatial scaling especially in mountainous areas, because of its influence on the lateral water redistribution, affecting water table, soil moisture and plant growth. Other factors including leaf area index (LAI) and elevation have small and additive effects on improving the spatial scaling between these two resolutions.

An evaluation of the hydrologic relevance of lateral flow in snow at hillslope and catchment scales

AbstractLateral downslope flow in snow during snowmelt and rain‐on‐snow (ROS) events is a well‐known phenomenon, yet its relevance to water redistribution at hillslope and catchment scales is not well understood. We used dye tracers, geophysical methods, and hydrometric measurements to describe the snow properties that promote lateral flow, assess the relative velocities of lateral flow in snow and soil, and estimate volumes of downslope flow. Results demonstrate that rain and melt water can travel tens of metres downslope along layers within the snowpack or at the snowpack base within tens of hours. Lateral flow within the snowpack becomes less likely as the snowpack becomes saturated and stratigraphic boundaries are destroyed. Flow along the base can be prevalent in all snowpack conditions. The net result of lateral flow in snow can be the deposition of water on the soil surface in advanced downslope positions relative to its point of origin, or direct discharge to a stream. Although both melt and ROS events can redistribute water to downslope positions, ROS events produced the most significant volumes of downslope flow. Direct stream contributions through the snowpack during one ROS event produced up to 12% of streamflow during the event. This can help explain rapid delivery of water to streams during ROS events, as well as anomalously high contributions of event water during snowmelt hydrographs. In catchments with a persistent snowpack, lateral redistribution of water within the snowpack should be considered a relevant moisture redistribution mechanism. Copyright © 2012 John Wiley & Sons, Ltd.

Land cover and water yield: inference problems when comparing catchments with mixed land cover

Abstract. Controlled experiments provide strong evidence that changing land cover (e.g. deforestation or afforestation) can affect mean catchment streamflow (Q). By contrast, a similarly strong influence has not been found in studies that interpret Q from multiple catchments with mixed land cover. One possible reason is that there are methodological issues with the way in which the Budyko framework was used in the latter type studies. We examined this using Q data observed in 278 Australian catchments and by making inferences from synthetic Q data simulated by a hydrological process model (the Australian Water Resources Assessment system Landscape model). The previous contrasting findings could be reproduced. In the synthetic experiment, the land cover influence was still present but not accurately detected with the Budyko- framework. Likely sources of interpretation bias demonstrated include: (i) noise in land cover, precipitation and Q data; (ii) additional catchment climate characteristics more important than land cover; and (iii) covariance between Q and catchment attributes. These methodological issues caution against the use of a Budyko framework to quantify a land cover influence in Q data from mixed land-cover catchments. Importantly, however, our findings do not rule out that there may also be physical processes that modify the influence of land cover in mixed land-cover catchments. Process model simulations suggested that lateral water redistribution between vegetation types and recirculation of intercepted rainfall may be important.

Ecosystem processes at the watershed scale: Mapping and modeling ecohydrological controls of landslides

Mountain watersheds are sources of a set of valuable ecosystem services as well as potential hazards. The former include high quality freshwater, carbon sequestration, nutrient retention, and biodiversity, whereas the latter include flash floods, landslides and forest fires. Each of these ecosystem services and hazards represents different elements of the integrated and co-evolved ecological, hydrological and geomorphic subsystems of the watershed and should be approached analytically as a coupled land system. Forest structure and species are important influences on the partitioning of precipitation, the lateral redistribution of water, runoff and sediment production, weathering and soil development. Forest regulation of hydrologic dynamics contributes to the development of patterns of soil pore pressure and slope instability during storms or snowmelt. The spatial patterns of root depth, structure and strength, developed by the below ground allocation of carbon in the forest canopy in response to limiting resources of water and nutrients, contributes to slope stability and drainage, and the maintenance of stomatal conductance linking water and carbon cycling. This in turn provides the photosynthate required to build leaf area, stem and root biomass. The linked ecological, hydrologic and geomorphic systems are characterized by specific catenary patterns that should be captured in any coupled modeling approach. In this paper we extend an ecohydrological modeling approach to include hydrologic and canopy structural pattern impacts on slope stability, with explicit feedbacks between ecosystem water, carbon and nutrient cycling, and the transient development of landslide potential in steep forested catchments. Using measured distributions of canopy leaf area index, and empirically modeled soil depth and root cohesion, the integrated model is able to generate localized areas of past instability without specific calibration or training with mapped landslides. As the model has previously been shown to simulate space/time patterns of coupled water, carbon and nutrient cycling, the integration of slope stability as a function of hydrologic, ecosystem and geomorphic processes provides the ability to closely link multiple ecosystem services with a unified approach.

The Hydrological Effects of Lateral Preferential Flow Paths in a Glaciated Watershed in the Northeastern USA

Despite observational evidence of lateral preferential flow paths in northeastern U.S. watersheds, their effects on the sources of runoff remain unclear. An intense field survey was undertaken during the 2007 growing season to determine the sources of stream runoff from a 2.51 km2 watershed in the Catskill Mountains, New York State. Lateral preferential flow paths are caused by groundwater springs and soil piping in this region. A two‐component hydrograph separation using δ18O showed that event water (rain water) was a significant source of runoff during nine rainfall events (from July to October). With these rainfall events, 14 to 37% of the volume and 18 to 49% of the peak streamflow was attributable to event water. Further, end‐member mixing analysis (EMMA), using δ18O, Si, and dissolved organic carbon (DOC), showed that saturated areas accounted for 2 to 24% of the total volume and 4 to 59% of peak streamflow but that groundwater was the dominant source of runoff volume during all events. Field surveys of saturated areas also suggested that near‐stream areas were insufficient to generate the observed stream chemistry during rainfall events larger than 8 mm. A connection with the hillside saturated areas was therefore required to explain the results of the hydrograph separations, which were corroborated by the timing of the transient (perched) groundwater and overland flow. The hydrometric measurements confirmed that hillside lateral preferential flow paths rapidly transported water to near‐stream saturation areas during runoff events under relatively dry antecedent conditions. A qualitative comparison with conventional techniques for distributing variable saturation areas (VSA) using surface topography and soil transmissivity (i.e., topographic index and soil topographic index), which do not consider the effects of lateral preferential flow paths, demonstrated that typical parameterizations (on the order of <10−1 m) would not have the spatial resolution to represent the measured lateral preferential flow paths (on the order of <10−3 m). Overall, the results suggest that the lateral redistribution of water from hillside areas reduces the influence of surface topography and channel topology on the sources of stream runoff, a finding that is consistent with recent ones from other landscapes where glacial soils have coevolved with the terrestrial hydrology.

Review of Dispersivities for Transport Modeling in Soils

The one‐dimensional convection–dispersion equation is often used to estimate the risk of nonpoint source groundwater contamination and the dispersivity in this equation is known to be a sensitive parameter for predicting the mass that leaches through the vadose zone to the groundwater. We derived a database of dispersivities from leaching studies in soils. Besides dispersivities, the database contains information about experimental parameters: transport distance, scale of the experiment, flow rate, boundary conditions, soil texture, pore water velocity, transport velocity, and measurement method. Dispersivities were found to increase with increasing transport distance and scale of the experiment. Considerably larger dispersivities were observed for saturated than for unsaturated flow conditions. No significant effect of soil texture on dispersivity was observed, but the interactive effects of soil texture, lateral scale of the experiment, and flow rate on dispersivity were significant. In coarse‐textured soils, lateral water redistribution may take place across relatively larger distances, which explains the larger dependency of dispersivity on lateral scale of the experiment in coarse‐ than in fine‐textured soils. The activation of large interaggregate pores may explain the increase in dispersivity with increasing flow rate in fine‐textured soils, which was not observed in soils with a coarser texture. The distribution of dispersivities was positively skewed and better described with a lognormal than a normal distribution. Different experimental factors explained 25% of the total variability of loge‐transformed dispersivities. The unexplained variance of the dispersivity was large and its coefficient of variation was 100%.

Soil moisture spatial variability in experimental areas of central Italy

Summary The knowledge of soil moisture spatial variability is an important issue for hydrological and climatic studies. This paper investigates the soil moisture spatial pattern obtained from spot measurements carried out in different experimental plots located in small basins of Central Italy. The near-surface volumetric soil moisture content was measured using a portable time domain reflectometer (TDR) over regular grids whose areas ranging in extension from 9 to 8800 m2. The sampling in each site was periodically repeated in order to investigate the moisture pattern as function of wetness conditions. The statistical properties of the measurements and the relation of the statistical parameters with the field-mean soil moisture content were analyzed. The results states that the probability distribution can be assumed normal for soil moisture in flat areas and for normalized soil moisture in gentle slope areas. A general decreasing trend of variance with increasing mean moisture content was observed. Moreover, considering the relationship between the field-mean water content and the coefficient of variation an optimal measurements number ranging from 15 to 35, in the worse case, was found for the study areas. The higher number was required for the site with more significant relief. These results were also confirmed through the geostatistical analysis and the correlation with the topographical attributes. Specifically, the correlation lengths were found increasing with the mean areal slope and the mean drainage direction and ranging between 10 and 15 m. This is corroborated through geostatistical analysis showing that the natural logarithm of the specific upslope area was the best spatial predictor for near-surface soil moisture when the water volume in the soil is higher. On this basis, the surface and subsurface lateral redistribution of water can be surmised fundamental in spatial organization of soil moisture.

The soil-geomorphic template and biotic change in arid and semi-arid ecosystems

The objective of this paper is to illustrate the concept of the soil-geomorphic template and its relationship to biotic change in arid and semi-arid regions. Such biotic change is typically accompanied by and linked to geomorphic change that involves soil, topography, and soil parent material, which together form the soil-geomorphic template. Soil is a factor in biotic change because it is the substrate that provides water, nutrients, anchorage for plants, and habitat for burrowing animals. Topography is a factor in biotic change because it influences local microclimate by means of elevation, lateral redistribution of water, and slope orientation. Soil parent material is a factor in biotic change because it provides the lithic inheritance from the geologic landscape that gives rise to soils with different particle size distribution (i.e. available water holding capacity) and nutrient status. Numerous linkages and feedback loops occur between the soil-geomorphic template, microclimate, vegetation, and animals. A perturbation in any of these factors can steer an ecosystem from one state to another. The integral relationship between the soil-geomorphic template and biotic change is an example of how biological and geological systems are coupled and co-evolve over long-term (Quaternary landscape evolution) and short-term (human-induced desertification) time-scales.

A spatial analysis of lateral and vertical fluxes of water associated with a perched watertable in a duplex soil

Rates of lateral and vertical movement of water were investigated when severe waterlogging occurred as a result of a perched watertable over a clay subsoil in a duplex soil on a low (1.6%) slope in Western Australia. The severity of waterlogging and both the lateral and vertical fluxes of water at the 1.4 ha site were very spatially variable. Lateral water movement occurred in response to topographical gradients in the soil surface and the depth of the clay layer. As a result of this lateral redistribution of water, the depth of the watertable was greater in some areas, causing higher rates of vertical water loss (1.6–1.7 mm/day) compared with the rest of the site (1.2–1.5 mm/day). Net gains or losses in water in experimental plots at the site due to lateral flow were 1–2 orders of magnitude smaller than vertical losses. Net lateral losses were less than 0.3 mm/day, but vertical losses ranged from 1.2 to 1.7 mm/day. We conclude that vertical drainage beneath crops growing on a duplex soil with a heavy clay subsoil can be significant when a head of water develops under climatic conditions conducive to waterlogging. In contrast, net lateral movement was generally small because of the low slope at the site (gradient of 0.016), and is likely to be insignificant at this site and other sites with duplex soils on similarly low slopes.

Observed spatial organization of soil moisture and its relation to terrain indices

We analyze the degree of spatial organization of soil moisture and the ability of terrain attributes to predict that organization. By organization we mean systematic spatial variation or consistent spatial patterns. We use 13 observed spatial patterns of soil moisture, each based on over 500 point measurements, from the 10.5 ha Tarrawarra experimental catchment in Australia. The measured soil moisture patterns exhibit a high degree of organization during wet periods owing to surface and subsurface lateral redistribution of water. During dry periods there is little spatial organization. The shape of the distribution function of soil moisture changes seasonally and is influenced by the presence of spatial organization. Generally, it is quite different from the shape of the distribution functions of various topographic indices. A correlation analysis found that ln(a), where a is the specific upslope area, was the best univariate spatial predictor of soil moisture for wet conditions and that the potential radiation index was best during dry periods. Combinations of ln(a) or ln(a/tan(β)), where β is the surface slope, and the potential solar radiation index explain up to 61% of the spatial variation of soil moisture during wet periods and up to 22% during dry periods. These combinations explained the majority of the topographically organized component of the spatial variability of soil moisture a posteriori. A scale analysis indicated that indices that represent terrain convergence (such as ln(a) or ln(a/tan(β))) explain variability at all scales from 10 m up to the catchment scale and indices that represent the aspect of different hillslopes (such as the potential solar radiation index) explain variability at scales from 80 m to the catchment scale. The implications of these results are discussed in terms of the organizing processes and in terms of the use of terrain attributes in hydrologic modeling and scale studies. A major limitation on the predictive power of terrain indices is the degree of spatial organization present in the soil moisture pattern at the time for which the prediction is made.