Humidity, soil moisture data key to fire, drought monitoring

Hotter, drier heat waves in western U.S. not surprising

Record-breaking temperatures across western states are straining tight electricity supplies. High night-time lows are discouraging residents from using fans and air conditioners less overnight.

Senior scientist at Lawrence Berkeley National Lab, Michael Wehner, estimates that once rare heat waves are now warmer by three to five degrees Fahrenheit in most of the United States.

University of California, Los Angeles (UCLA) climate scientist, Karen McKinnon, has further observed that specific humidity has decreased by about 22 percent on the West’s hottest, driest days "after accounting for rising temperatures" between 1973 and 2019. Data from weather stations in California and Nevada alone show specific humidity dropped by 33 percent over the same 40-year period.

She also identified that absolute humidity has been getting lower – mostly over the last 20 years, since 2000.

Although less humidity is relatively less threatening to human health via body cooling, the trend poses greater fire risk and environmental impact. UCLA climate scientist Daniel Swain explained that since hotter air can hold more water, the increasing vapor pressure deficit will continue to be a major driver of more intense wildfires. The widening gap between the actual amount of moisture in the air and the potential capacity means ‘the landscape is drying out faster than ever’.

Additionally UCLA researchers have noted a trend in decreasing summer soil moisture. Plants have less water to transpire, or release, into the air. Lower soil moisture measurements are also indicators of less snowpack, more evaporation, and warmer temperatures in winter and spring seasons.

In Colorado, the Aspen Global Change Institute tracks precipitation, temperatures and soil moisture levels at higher elevations. One goal is to gauge impacts of a warming climate on soil moisture and ecology in the Roaring Fork Valley. The soil moisture is measured at two-, eight- and 20-inch levels.

soil moisture data at varying depths reveal impacts from rainfall, drought, and snowpack | image credit Aspen Global Change Institute

Community science manager, Elise Osenga, described each depth’s importance. At two inches (5.08 cm), surface temperatures reveal the weather’s effect on topsoil and shallow root systems, whereas 20 inches (50 cm) below the surface show snowpack runoff. The intermediate eight-inch depth (20.3 cm) shows effects of drought and rainstorms on soil health as at least one-half inch of rainfall is needed to reach that depth.

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You can read McKinnon’s study, “Hot extremes have become drier in the United States Southwest,” published on 6/17/2021 by the nature climate change journal.

Learn more about Soil Moisture Data Services provided by KISTERS in partnership with VanderSat. Get high-resolution satellite datasets for depths up to 2-inches (5 cm) for root zone options ranging from 10 cm to 40 cm, over an area as granular as 100 m2 (0.025 sq acre).

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