The Global Soil Moisture Data Bank

Abstract

Soil moisture is an important variable in the climate system. Understanding and predicting variations of surface temperature, drought, and flood depend critically on knowledge of soil moisture variations, as do impacts of climate change and weather forecasting. An observational dataset of actual in situ measurements is crucial for climatological analysis, for model development and evaluation, and as ground truth for remote sensing. To that end, the Global Soil Moisture Data Bank, a Web site (http://climate.envsci.rutgers.edu/soil_moisture) dedicated to collection, dissemination, and analysis of soil moisture data from around the globe, is described. The data bank currently has soil moisture observations for over 600 stations from a large variety of global climates, including the former Soviet Union, China, Mongolia, India, and the United States. Most of the data are in situ gravimetric observations of soil moisture; all extend for at least 6 years and most for more than 15 years. Most of the stations have grass vegetation, and some are agricultural. The observations have been used to examine the temporal and spatial scales of soil moisture variations, to evaluate Atmospheric Model Intercomparison Project, Project for Intercomparison of Land-Surface Parameterization Schemes, and Global Soil Wetness Project simulations of soil moisture, for remote sensing of soil moisture, for designing new soil moisture observational networks, and to examine soil moisture trends. For the top 1-m soil layers, the temporal scale of soil moisture variation at all midlatitude sites is 1.5 to 2 months and the spatial scale is about 500 km. Land surface models, in general, do not capture the observed soil moisture variations when forced with either model-generated or observed meteorology. In contrast to predictions of summer desiccation with increasing temperatures, for the stations with the longest records summer soil moisture in the top 1 m has increased while temperatures have risen. The increasing trend in precipitation more than compensated for the enhanced evaporation.

Changes in the phase of the annual cycle of surface temperature

“…IPCC Fourth Assessment Report model runs disagree with each other on the sign of recent soil moisture trends and show little skill at explaining the (sparse) observations 36 . The few available long-term measurements suggest increased soil moisture over the latter part of the twentieth century 37,38 , which is inconsistent with the thermal mass hypothesis, although we observe that drought reconstructions 39 indicate these observations may not be representative of continental-scale variations. The paucity of records with more than 40 years of data prohibits a more detailed comparison.…”

Changes in Australian pan evaporation from 1970 to 2002

“…At energy-limited sites, a decrease in pan evaporation, at constant rainfall, implies that actual evaporation will decrease and runoff and/or soil moisture will increase. Importantly, all of these predicted changes have been observed in Russia over the last 50 years (Robock et al, 2000;Golubev et al, 2001;Peterson et al, 2002). Conversely, in a water-limited environment, actual evaporation is limited by available water and not energy, and changes in actual evaporation are dominated by changes in rainfall (Budyko, 1974;Choudhury, 1999;Milly and Dunne, 2002).…”

Soil moisture-based study of the variability of dry-wet climate and climate zones in China

“…Additionally, there has been research on the vertical changes of soil moisture and the spatialtemporal relations between precipitation and temperature [16,17]. However, there has been less observational research on the dry-wet climate and boundary dynamics of climate zones on regional scales due to the unavailability of long-term in situ soil moisture observations [18]. Furthermore, although remote sensing and monitoring have the advantage of global coverage, the soil moisture data obtained are strongly dependent on the retrieval model used and are constrained by a limited observational depth.…”