In an ideal world, measurements of soil moisture content — at several levels from the surface of the ground to five feet below or deeper — would be available on a daily basis for every backyard and field across the United States. This type of observation network would give us a good idea of how dry or wet the ground is and help tremendously with drought monitoring. Such a national soil moisture observation network doesn't exist. At present, a few hundred soil moisture stations are scattered around the country. So, water balance models are run on a gridded spatial scale in order to get an idea of what the national soil moisture conditions are. Examples of soil moisture water balance models include the "Leaky Bucket", North American Land Data Assimilation System (NLDAS), and VegET models. Water balance models typically use precipitation as the water supply component. They calculate a water demand component (evapotranspiration) using temperature and other variables such as humidity, wind speed, and insolation (solar energy). Then they calculate fluxes (how water and energy change over time and space) to estimate things like soil moisture, soil temperature, snow water content, and stream runoff. Some of the models use station soil moisture observations and satellite observations of surface wetness as "ground truth" for calibration. While not perfect, using a variety of models gives us a good idea of where soils are drying across the country, especially in areas where no soil moisture observations exist.
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