[1] The contrast between the point-scale nature of current ground-based soil moisture instrumentation and the ground resolution (typically >102 km2) of satellites used to retrieve soil moisture poses a significant challenge for the validation of data products from current and upcoming soil moisture satellite missions. Given typical levels of observed spatial variability in soil moisture fields, this mismatch confounds mission validation goals by introducing significant sampling uncertainty in footprint-scale soil moisture estimates obtained from sparse ground-based observations. During validation activities based on comparisons between ground observations and satellite retrievals, this sampling error can be misattributed to retrieval uncertainty and spuriously degrade the perceived accuracy of satellite soil moisture products. This review paper describes the magnitude of the soil moisture upscaling problem and measurement density requirements for ground-based soil moisture networks. Since many large-scale networks do not meet these requirements, it also summarizes a number of existing soil moisture upscaling strategies which may reduce the detrimental impact of spatial sampling errors on the reliability of satellite soil moisture validation using spatially sparse ground-based observations.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2011RG000372

Upscaling sparse ground‐based soil moisture observations for the validation of coarse‐resolution satellite soil moisture products

Satellite remote sensing technology has been widely used to estimate surface soil moisture. Numerous efforts have been devoted to develop global soil moisture products. However, these global soil moisture products, normally retrieved from microwave remote sensing data, are typically not suitable for regional hydrological and agricultural applications such as irrigation management and flood predictions, due to their coarse spatial resolution. Therefore, various downscaling methods have been proposed to improve the coarse resolution soil moisture products. The purpose of this paper is to review existing methods for downscaling satellite remotely sensed soil moisture. These methods are assessed and compared in terms of their advantages and limitations. This review also provides the accuracy level of these methods based on published validation studies. In the final part, problems and future trends associated with these methods are analyzed.

/pdf/a-review-of-spatial-downscaling-of-satellite-remotely-sensed-ti94yv66sd.pdf

A review of spatial downscaling of satellite remotely sensed soil moisture

A critical review of soil moisture measurement

Soil moisture is an essential climate variable influencing land-atmosphere interactions, an essential hydrologic variable impacting rainfall-runoff processes, an essential ecological variable regulating net ecosystem exchange, and an essential agricultural variable constraining food security. Large-scale soil moisture monitoring has advanced in recent years, creating opportunities to transform scientific understanding of soil moisture and related processes. These advances are being driven by researchers from a broad range of disciplines, but this complicates collaboration and communication; and, for some applications, the science required to utilize large-scale soil moisture data is poorly developed. In this review, we describe the state of the art in large-scale soil moisture monitoring and identify some critical needs for research to optimize the use of increasingly available soil moisture data. We review representative examples of (i) emerging in situ and proximal sensing techniques, (ii) dedicated soil moisture remote sensing missions, (iii) soil moisture monitoring networks, and (iv) applications of large-scale soil moisture measurements. Significant near-term progress seems possible in the use of large-scale soil moisture data for drought monitoring. Assimilation of soil moisture data for meteorological or hydrologic forecasting also shows promise, but significant challenges related to spatial variability and model structures remain. Little progress has been made in the use of large-scale soil moisture observations within the context of ecological or agricultural modeling. Opportunities abound to advance the science and practice of large-scale soil moisture monitoring for the sake of improved Earth system monitoring, modeling, and forecasting.

/pdf/state-of-the-art-in-large-scale-soil-moisture-monitoring-5fvfxwavmi.pdf

State of the Art in Large‐Scale Soil Moisture Monitoring

Hydrologic research is a very demanding application of fiber-optic distributed temperature sensing (DTS) in terms of precision, accuracy and calibration. The physics behind the most frequently used DTS instruments are considered as they apply to four calibration methods for single-ended DTS installations. The new methods presented are more accurate than the instrument-calibrated data, achieving accuracies on the order of tenths of a degree root mean square error (RMSE) and mean bias. Effects of localized non-uniformities that violate the assumptions of single-ended calibration data are explored and quantified. Experimental design considerations such as selection of integration times or selection of the length of the reference sections are discussed, and the impacts of these considerations on calibrated temperatures are explored in two case studies.

/pdf/calibrating-single-ended-fiber-optic-raman-spectra-as79hnt108.pdf

Calibrating single-ended fiber-optic Raman spectra distributed temperature sensing data.

Accurate methods are needed to measure changing soil water content from meter to kilometer scales. Laboratory results demonstrate the feasibility of the heat pulse method implemented with fiber optic temperature sensing to obtain accurate distributed measurements of soil water content. A fiber optic cable with an electrically conductive armoring was buried in variably saturated sand and heated via electrical resistance to create thermal pulses monitored by observing the distributed Raman backscatter. A new and simple interpretation of heat data that takes advantage of the characteristics of fiber optic temperature measurements is presented. The accuracy of the soil water content measurements varied approximately linearly with water content. At volumetric moisture content of 0.05 m3/m3 the standard deviation of the readings was 0.001 m3/m3, and at 0.41 m3/m3 volumetric moisture content the standard deviation was 0.046 m3/m3. This uncertainty could be further reduced by averaging several heat pulse interrogations and through use of a higher?performance fiber optic sensing system.

/pdf/feasibility-of-soil-moisture-monitoring-with-heated-fiber-4hpn8jet9q.pdf

Feasibility of soil moisture monitoring with heated fiber optics

http://www.geology.wisc.edu/%7Edavem/abstracts/08-13.pdf

Numerical simulation of the paleohydrology of glacial Lake Oshkosh, eastern Wisconsin, USA

Recognizing that elevated stream temperatures can harm aquatic organisms, regulatory agencies have begun to enforce thermal total daily maximum load (TMDL) limits on wastewater discharges. Infiltration discharge, where treated wastewater is allowed to infiltrate into the soil and then percolate toward the stream or river, represents a potential method for meeting temperature requirements and for supplementing flow. This study combines observed and simulated temperature data to assess the efficacy of using infiltration discharge to meet effluent temperature limits. Observational data collected during operation of a 0.15-ha pilot-scale infiltration discharge system revealed the pattern of groundwater heating beneath the site by the wastewater, with the largest temperature increases occurring near the infiltration point. Numerical simulations used to examine the long-term groundwater response to operation of a 5.5-ha large-scale infiltration wetland system confirmed this trend and demonstrated how mu...

Christopher T. Gregory

Papers

Feasibility of soil moisture monitoring with heated fiber optics

Numerical simulation of the paleohydrology of glacial Lake Oshkosh, eastern Wisconsin, USA

Evaluation of Infiltration Discharge as a Strategy to Meet Effluent Temperature Limits