Soil Moisture Measurement for Ecological and Hydrological Watershed‐Scale Observatories: A Review

Abstract

Over the oceans, approximately 90% of net radiation produces evaporation (Budyko, 1974), primarily in the tropics. Over continents, net radiation heats the surface, evaporates water from water bodies or moist soils, or provides plants with energy to remove water from soils (Pitman, 2003; Istanbulluoglu and Bras, ANALYSISAt the watershed scale, soil moisture is the major control for rainfall-runoff response, especially where saturation excess runoff processes dominate. From the ecological point of view, the pools of soil moisture are fundamental ecosystem resources providing the transpirable water for plants. In drylands particularly, soil moisture is one of the major controls on the structure, function, and diversity in ecosystems. In terms of the global hydrological cycle, the overall quantity of soil moisture is small, ∼0.05%; however, its importance to the global energy balance and the distribution of precipitation far outweighs its physical amount. In soils it governs microbial activity that aff ects important biogeochemical processes such as nitrifi cation and CO 2 production via respiration. During the past 20 years, technology has advanced considerably, with the development of diff erent electrical sensors for determining soil moisture at a point. However, modeling of watersheds requires areal averages. As a result, point measurements and modeling grid cell data requirements are generally incommensurate. We review advances in sensor technology, particularly emerging geophysical methods and distributed sensors, aimed at bridging this gap. We consider some of the data analysis methods for upscaling from a point to give an areal average. Finally, we conclude by off ering a vision for future research, listing many of the current scientifi c and technical challenges.

Passive L-Band Microwave Remote Sensing of Organic Soil Surface Layers: A Tower-Based Experiment

“…In addition, the largest variations with increasing frequency were observed under high clay and organic matter contents. Kelleners et al [37] also observed in dispersive clay soils a frequency dependence of the dielectric permittivity below 500 MHz with lower values of dielectric permittivity at higher frequencies as observed in the current study (see also [38]). Similar effects might indeed be expected in organic soils as in clay soils, as the organic constituents have also large specific areas with high surface charges.…”

Nature's neutron probe: Land surface hydrology at an elusive scale with cosmic rays

“…The measured values of key land surface variables, such as snow water equivalent depth, soil water content and biomass, depend strongly on spatial scale of observation, but upscaling small-volume and downscaling large-volume measurements to the element size of predictive numerical models has remained problematic [Blöschl, 2001]. This problem is compounded by the gap between the two main sources of observational data: large-scale remote sensing images with a footprint extending to tens of kilometers and penetration depth of millimeters to centimeters, and invasive measurements of water content at a point in a spatially variable field [Robinson et al, 2008]. The crucial need for hydrologic observations that correspond to the water content at the scale of an irrigated field, a small watershed or a hydrometeorologic model element has not been satisfied with conventional technologies.…”

Soil Moisture Data for the Validation of Permafrost Models Using Direct and Indirect Measurement Approaches at Three Alpine Sites

“…For the in situ measurements of soil moisture and its temporal evolution numerous well established direct and indirect methods are available (e.g., Hillel, 2004;Robinson et al, 2008;Vereecken et al, 2008). Among them only few were tested in terrains undergoing seasonal or permanent frozen conditions.…”