Relating temperature extremes and droughts to evaporation estimates from SMAP passive microwave soil moisture retrievals

With the presence of El Niño in 2015, soil moisture deficits, droughts and heat waves are observed over central and northern Europe, Western USA, southeastern parts of Asia and Equatorial Africa. In many European countries, e.g. France, Austria, Belgium and Poland, the continuing drought and high temperatures lead to crop loss. Water and energy fluxes at the land surface, mainly driven by soil moisture and temperature state of the land surface, impact weather and can lead to extremes like mega heat waves (Miralles et al., 2014) and droughts (Teuling et al., 2013). Typically, droughts and soil moisture deficits are related to precipitation deficits and the effect of evaporation is not yet well understood. Depletion of soil moisture leads to a decrease in evaporation and subsequently a reduction in evaporative cooling, enabling the increase in air temperatures leading to heat waves (Miralles et al., 2014). On the other side, increase of evaporation and atmospheric evaporative demand is observed during drought periods, amplifying storage anomalies (Seneviratne et al., 2012; Teuling et al., 2013). Evaporation is a key parameter linking the water, energy and carbon cycles over land, and yet its effect on extremes is not well understood.

In this presentation we demonstrate the use of NASA's new Soil Moisture Active Passive passive microwave observations, in situ precipitation data and a simple water balance model to directly estimate evaporation. Similar as done by Brocca et al. (2013) quantitative estimates of evaporation are obtained from soil moisture data directly, by solving the water balance equation for evaporation and estimating drainage using conditional sampling of soil moisture (Salvucci, 2001). Evaporation estimates are compared to in situ measurements from lysimeters and eddy covariance stations over three different locations.

This approach enables the investigation of dynamics in evaporation after rainfall events. The timing of the transition between the different stages of evaporation is assessed, more precisely the transition from stage 1 to stage 2 evaporation: The first stage is the energy limited stage, where the evaporation equals the atmospheric demand and no water stress exists. Stage two is the falling rate stage, where evaporation is limited by soil moisture diffusion in the soil. With the start of stage 2 evaporation decreases due to soil moisture deficits. When these conditions persist, this leads to an increase in sensible heat flux and subsequently air temperatures, water stress in plants and can over time lead to droughts and heat waves.

By estimating the duration of stage 1 and stage 2 evaporation during continuous drying events in an inter-storm period for a certain location, feedbacks between evaporation and the extreme conditions of droughts and heat waves can be investigated. The number of days where stage 2 evaporation occurs in 2015 is compared to earlier years. It is expected that, in areas which were affected by heat waves and droughts, more days with stage 2 evaporation occur. In addition, the evaporation is compared to air temperature to assess the interaction between evaporation and temperature. The results of this study help understand the links between the water, energy and carbon cycle. Furthermore, it can provide additional information for risk management in predicting droughts. When applied to a smaller scale of 500 m to 3 km, using e.g. soil moisture data from SMAP radar or Sentinel-1, it can provide crucial information in preventing agricultural losses.

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Miralles, D.G., Teuling, A.J., van Heerwaarden, C.C., Vilà-Guerau de Arellano, J., 2014. Mega-heatwave temperatures due to combined soil desiccation and atmospheric heat accumulation. Nat. Geosci. 7, 345–349. doi:10.1038/ngeo2141 Salvucci, G.D., 2001. Estimating the moisture dependence of root zone water loss using conditionally averaged precipitation. Water Resour. Res. 37, 1357–1365. doi:10.1029/2000WR900336

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Teuling, A.J., Van Loon, A.F., Seneviratne, S.I., Lehner, I., Aubinet, M., Heinesch, B., Bernhofer, C., Grünwald, T., Prasse, H., Spank, U., 2013. Evapotranspiration amplifies European summer drought. Geophys. Res. Lett. 40, 2071–2075. doi:10.1002/grl.50495