Science Center Objects

Evapotranspiration (ET) rates – the combination of evaporation from soil and transpiration from plants – are powerful tools in the management of scarce water resources.

Taken alone, however, those rates leave unanswered questions about water sources, questions that can offer guidance and insight on challenging decisions in areas reliant on irrigation.

Researchers at EROS work to offer that insight through the study of water balance and soil moisture.

The foundation of the work is a simple but robust methodology to partition ET rates between rainfall – “green water” – and irrigation from lakes, rivers, reservoirs and groundwater sources – “blue water.”

The methodology uses spatially-explicit precipitation, soil properties, vegetation phenology using Normalized Difference Vegetation Index (NDVI), Land Surface Temperature (LST) and other weather datasets to understand the dynamics of ET rates from green water and blue water, analyze sources of ET for different land cover types and to quantify partitioned ET rates for irrigated cropland in the conterminous United States.

Illustration of green water and blue water evapotranspiration

An illustration of Green Water and Blue Water Evapotranspiration. (Public domain.)

Two models are brought to bear to reach those goals. The first uses a root-zone Water Balance ET (WBET) approach driven by gridded precipitation, which allows for large-scale characterization of ET at 1-kilometer resolution. The parameters in WBET are not influenced by irrigation water applications, thus capturing true measurements of only green water ET.

The second model uses the Energy-balance ET approach (EBET), which is driven by LST. This approach implicitly accounts for the impact of water stress (from rainfall and/or irrigation) regardless of the source of moisture, hence capturing the sum of green and blue water ET.

When these two datasets (WBET and EBET) are inter-calibrated and validated against an independent external data source, researchers at EROS can offer high-resolution (1 km) estimates and datasets that separate green water and blue water ET rates for all landscapes. Current and future work is focused on using Landsat (30 m) data to produce blue and green water ET maps for field scale analysis.

The information gleaned about the relative contributions of different water sources to ET rates provide a deeper understanding of the need for and value of irrigation across various crop types, regions and land cover classes.

With groundwater being depleted for crops at unsustainable rates, that understanding is crucial to planning for the long-term viability of agriculture and food production systems and to the ecosystems that rely on the same groundwater for viability.

Evaluation of global soil moisture data

EROS’ water balance and soil moisture modeling can also validate and improve confidence in other potential sources of drought monitoring data.

Precipitation is the best observed hydrologic variable, but precipitation alone cannot adequately characterize a drought. Other agro-hydrologic parameters such as LST, NDVI and ET have been used in several standard drought indices, but each is an expression of the key hydrologic variable: Soil moisture.

NASA’s Soil Moisture Active Passive (SMAP) satellite, launched in 2014, offers a global look at soil moisture through remote sensing. It promises to be a valuable tool, as no comprehensive, national network of soil moisture monitoring instruments exists to provide seamless information on soil moisture status. SMAP measures moisture in the first five centimeters of soil and freeze/thaw state on a global scale, with a revisit time of 2-3 days.

Researchers at EROS worked to evaluate SMAP soil moisture (SSM) data for drought monitoring in the rangelands of the U.S. High Plains. The study used U.S. Climate Reference Network (USCRN) observations from sites in Texas and Oklahoma and basin-scale estimates from the Vegetation Evapotranspiration (VegET) model to validate preliminary data from SMAP. The resulting study showed a high level of agreement with SMAP soil moisture (SSM) and the modeled estimates.

The results demonstrated the potential for using SSM for drought monitoring studies in rangeland ecosystems.

Acceleration of Aquifer Storage and Recovery

Water balance modeling also holds value for resource managers who work to preserve and recharge groundwater supplies.

Aquifers are an important source of water in the Middle East and North Africa (MENA) region, but aquifers suffer from high pumping rates and low recharge rates. Trapping rainfall, runoff and treated wastewater in places with high potential for aquifer recharge through methods collectively known as Aquifer Storage and Recovery (ASR) can bolster water security in areas where population growth, irrigation or persistent drought have placed pressure on available water stores.

ASR is underutilized in the MENA region because of the high cost of site identification and the difficulties matching ASR technology to site conditions.

Currently, EROS scientists are working to identify and map high-potential recharge sites in areas of Jordan, Israel, the West Bank and Lebanon. The information will be offered to local decision-makers, who can then decide where to place water-trapping barriers.

First, satellite-based precipitation products are used to generate improved regional rainfall dataset. Then VegET is used to track the pathways and magnitude of rainfall in soil-vegetation system is used to model runoff in the study region. Finally, potential sites for ASR are selected based on the analysis of 32 years of modelled runoff patterns and other on-the-ground criteria.

The work is a partnership between the USGS and universities, universities, research centers, non-governmental organizations, and government agencies, including: American University of Beirut; An-Najah National University; Arab Water Council; Hydrology.nl; Jordan Ministry of Water and Irrigation; Jordan National Center for Research & Development; Lebanon Ministry of Energy and Water; and Palestinian Water Authority.