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REVIEW AND ANALYSIS

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

^a Dep. of Geophysics, 397 Panama Mall, Stanford Univ., Stanford, CA 94305-2215
^b Dep. of Crop and Soil Sciences, Washington State Univ., Pullman, WA 99164
^c Dep. of Land, Air and Water Resources, Univ. of California, Davis, CA 95616
^d Dep. of Agronomy, Iowa State Univ. of Science and Technology, Ames, IA 50011-1010
^e Dep. of Plants, Soils and Climate, Utah State Univ., Logan, UT 84322-4820
^f Dep. of Civil & Architectural Engineering, Univ. of Wyoming, Laramie, WY 82071
^g Dep. of Biological and Ecological Engineering, Oregon State Univ., Corvallis, OR 97331-3906
^h Dep. of Plant and Soil Sciences, Univ. of Kentucky, Lexington, KY 40546-0091
ⁱ current address: Dep. of Food Production, Univ. of the West Indies, Trinidad and Tobago
^* Corresponding author (darearthscience{at}yahoo.com).

Received 16 August 2007.

At 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 affects important biogeochemical processes such as nitrification and CO₂ production via respiration. During the past 20 years, technology has advanced considerably, with the development of different 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 offering a vision for future research, listing many of the current scientific and technical challenges.

Abbreviations: ATV, all-terrain vehicle • CASMM, catchment average soil moisture monitoring • DC, direct current • DPHP, dual-probe heat pulse method • EM, electromagnetic • EMI, electromagnetic induction • GPR, ground penetrating radar • H, horizontally polarized • HPP, heat pulse probe • LIDAR, light detection and ranging • MFHPP, multifunctional heat pulse probe • NMR, nuclear magnetic resonance • SCAN, soil analysis climate network • T₁, spin-lattice relaxation time in NMR • T₂, spin-spin relaxation time in NMR • TDR, time domain reflectometry • TDT, time domain transmission • V, vertically polarized