Evapotranspiration

"The process of transform- ing soil water into water vapor through the combination of evaporation from the soil surface and plant water use" (Virginia Polytechnic Institute and State University, n.y.) 

Sources

Virginia Polytechnic Institute and State University. "A Glossary  of Water-Related Terms", by Brian Benham. Publication 442-760

Related Content

Article

Making Irrigation Decisions from Space: Utilizing a New Thermal Remote Sensing Satellite to Protect Food Security

Have you ever questioned if there would be enough food at the store for everyone in your community? If you frequent a grocery store or market, probably not. Every Sunday I go to the grocery store with a list of foods I’ll need for the week and no contingency plan for what to do if there isn’t enough. If something is out of stock, I’ll just go to the next grocery store down the road. We take it for granted that certain foods will always be available for us to purchase. However, many people do not have the luxury of a reliable food source.

Space technologies in the detection, monitoring and management of groundwater

Global groundwater supplies

Groundwater accounts for 30% of Earth’s freshwater resources (Shiklomanov 1993) (Figure 1) and is estimated to globally provide 36% of potable water, 42% of irrigation water, and 24% of industrial water – indicating its significant value (Global Environment Facility 2021). Groundwater affords a host of benefits, from providing better protection against drought and microbiological contamination than surface waters, to being generally low cost and accessible to many users.

Interview with Victor Pellet, CNES PostDoc, Paris Observatory

Describe experience relating to water and space technologies

I grew up in a country (France) where water is freely available. The drought in 2003 was considered a one-time event. I had no single lesson on climate change at school. Despite this background, I was raised aware of the links between social and environmental inequality on a global scale.

Interview with Victor Pellet, CNES PostDoc, Paris Observatory

Describe experience relating to water and space technologies

I grew up in a country (France) where water is freely available. The drought in 2003 was considered a one-time event. I had no single lesson on climate change at school. Despite this background, I was raised aware of the links between social and environmental inequality on a global scale.

Capacity Building and Training Material

ARSET - Applications of remote sensing-based evapotranspiration data products for agricultural and water resource management

Overview

Evapotranspiration (ET) is the process by which the land surface returns water to the atmosphere in the form of moisture. ET is a very important part of the water cycle in the Earth system. It is the sum of evaporation from bare soil and transpiration from vegetation. For a given watershed, the supply of water from precipitation, surface and groundwater can be depleted via ET. Therefore, estimating the amount of ET is crucial for calculating the overall water budget and for effective water management.

ARSET - Water resource management using NASA Earth science data

Overview:

This online course covers precipitation (rainfall and snow fraction), soil moisture, evapotranspiration, runoff and streamflow, groundwater, and lake level heights. Participants are introduced to a number of NASA data products.

Objective:

Participants will be able to use NASA remote sensing observations and land-atmosphere models to: 

ARSET - Applications of remote sensing to soil moisture and evapotranspiration

Overview:

NASA's Soil Moisture Active Passive (SMAP) Satellite Mission is providing new soil moisture data, and modelling frameworks are providing new evapotranspiration data. This webinar series is intended to help participants learn about NASA soil moisture and evapotranspiration products and how to access and apply them for water resource management. Throughout the sessions, participants will learn how to monitor and manage water resources with techniques learned in training. The series begins with an introduction to satellite missions and useful data sets.

Event

Local Perspectives Case Studies

Stakeholder

University of Natural Resources and Life Sciences Vienna

Founded in 1872, the Universität für Bodenkultur Wien / University of Natural Resources and Life Sciences, Vienna, also known by its acronym "BOKU" is an education and research institution for renewable resources in Vienna, Austria. Today, BOKU comprises of 15 departments located at two sites in Vienna and one in Lower Austria, as well as several external research and teaching facilities in Austria. There are currently approximately 11000 students enrolled at BOKU in study courses at the bachelor, master, and doctoral levels.

Govind Ballabh Pant University of Agriculture and Technology Pantnagar

G. B. Pant University of Agriculture and Technology, also known as Pantnagar University, is the first agricultural university in India. The University lies in the campus town of Pantnagar in Kichha Tehseel and in the district of Udham Singh Nagar, Uttarakhand. The university is regarded as the harbinger of the Green Revolution in India. Pantnagar University is regarded as a significant force in the development and transfer of High Yielding Variety of seeds and related technology.

Université Chouaib Doukkali

The Chouaib Doukkali University (CDU) [www.ucd.ac.ma] in El Jadida, Morocco was founded in 1985. It is a public institution of higher education and scientific research. At present, in the Chouaib Doukkali there are 6 faculties, and has more than 507 teachers, 255 administrators, and more than 25 000 students. Training is provided for bachelor degree and master degree. In terms of research, the University has established two centers for doctoral studies, with 25 laboratories involving 82 research teams.

Person

Space-based Solution

Addressed challenge(s)

The ecohydrological trade-off in Nepal’s Middle Hills: mapping spring decline and groundwater loss in community forests through space-based solutions

Collaborating actors (stakeholders, professionals, young professionals or Indigenous voices)
Suggested solution

This solution combines multi-source satellite remote sensing, ecohydrological modeling, and community science to address spring decline and water insecurity caused by afforestation and land-use changes in Nepal's Middle Hills. The integrated approach offers a pathway to scientifically informed, community-driven forest and water management.

  1. Satellite Data Fusion: The first core strategy involves fusing multi-temporal and multi-sensor satellite data to assess vegetation trends, hydrological changes, and potential spring recharge zones.
  • Vegetation Monitoring: Time-series NDVI, EVI, and LAI are derived using Sentinel-2, Landsat, NOAA and MODIS. These indices help detect vegetation growth trends and assess forest types based on phenological signatures.
  • Cloud Mitigation: Nepal’s rugged terrain and monsoon conditions create persistent cloud cover challenges. While no perfect cloud-removal technique exists, we aim to apply machine learning and established algorithms like CLAY3 or Fmask to improve data quality for vegetation and land cover analysis.
  • Hydrological Metrics: ET using MODIS, Soil moisture is mapped using SMAP data, downscaled using terrain parameters such as slope and elevation. GRACE data informs groundwater trends. Satellite-based precipitation datasets are validated against DHM station data to compensate for missing or sparse in-situ observations.

 

  1. RHESSys Ecohydrological Simulation: The RHESSys model simulates the complex interactions between vegetation, soil moisture, surface and subsurface water flow, and groundwater storage. The model is run in growth mode to evaluate how forest type changes influence spring discharge.
  • MODIS ET and Sentinel-1 soil moisture serve as validation inputs.   
    The model provides spatial outputs including groundwater depth, lateral flow, and baseflow dynamics—critical for delineating micro-watersheds and assessing recharge efficiency.

 

  1. Recharge Zone and Spring Hotspot Mapping: Topographic indices such as TWI (Topographic Wetness Index) and HAND (Height Above Nearest Drainage), derived from ASTER DEMs, are used to identify spring recharge zones.
  • These zones are further validated using RHESSys outputs, satellite data layers, and available field measurements.
  • Machine learning (e.g., Random Forest) and participatory mapping help cross-check locations of active and declining springs.
  • The resulting maps guide protection measures and afforestation policies targeting hydrologically sensitive areas.

 

  1. Field Validation and Community Co-Design: Local participation through spring monitoring and mapping ensures the integration of indigenous knowledge with scientific analysis. Field measurements validate model predictions and support community-driven management strategies.

 

Requirements

Data

  • Remote sensing: NDVI, EVI, LAI, Evapotranspiration (MODIS), Soil Moisture (SMAP), satellite-based precipitation, terrestrial Groundwater storage (GRACE), Land Use Land Cover, DEM (ASTER).
  • In-situ: Precipitation and temperature data from DHM Nepal

Software

  • Google Earth Engine (cloud computing, satellite data analysis)
  • GRASS GIS and QGIS (terrain analysis, TWI, HAND)
  • RHESSys (eco-hydrological modeling)
  • R/Python (statistical modeling, ML integration)
  • LEAF Toolbox for LAI/phenology

Physical Requirement

  • Cross-validation of spring locations and groundwater depth measurements through field visits.

Priority Support Areas: To realize objectives, we seek support in the following areas:

  • High-Resolution and Cloud-Free Satellite Data: Technical assistance in accessing and processing Sentinel-1 SAR, Sentinel-2, and Landsat data, and applying cloud-removal algorithms.
  • Downscaling Remote Sensing Products: Assistance in refining MODIS-based NDVI, EVI, LAI, and phenological indicators using auxiliary terrain and land cover datasets.
  • Integration of Hydrological and Remote Sensing Data: Guidance on synchronizing outputs from RHESSys with MODIS, SMAP, and GRACE datasets for robust cross-validation.
  • Mapping Recharge and Spring Zones: Technical support in combining terrain indices with RHESSys-derived metrics to map spring recharge zones and inform land-use planning.

 

Outline steps for a solution

Phase 1: Satellite Analysis and Vegetation Mapping (completed)

 

Phase 2: GIS and Terrain Modeling (In Progress)

  • Use DEM, LULC, TWI, HAND for recharge/discharge mapping.   
    Analyze terrain factors (slope, curvature, aspect, valley) for moisture prediction.

 

Phase 3: Hydrological Simulation and Analysis (To Do)

  • Run RHESSys in growth mode to simulate hydrological-vegetation dynamics.
  • Validate against spring discharge logs.
  • Output: groundwater depth, saturation deficit, soil moisture, flow trends, seasonal water availability.

 

Phase 4: Community Co-Design and Policy Translation (To Do)

  • Share vulnerability maps with CFUGs and municipal planners.
  • Recommend native species afforestation rather than heavily water dependent species.
  • Promote thinning, selective logging, litter removal, etc. (forest management plant).
  • Water availability zone mapping for settlement

 

RHESSys model

Note: "The full code and comprehensive instructions for running the model are provided in this repository."

Results

  • Preliminary analysis shows increasing trends in both evapotranspiration and LAI, indicating higher water consumption by vegetation and less water available for downstream use. 
  • Mapped recharge zones and high-risk spring areas.
  • Groundwater storage trends over time and identification of water-available zones suitable for settlement planning.
  • Spatial maps and hydrologic models to support forest and water governance.
  • Policy briefs on forest-water tradeoffs and spring recovery.
  • Community awareness on how certain forest types and species are accelerating water loss and increasing water stress for downstream communities, prompting migration.
  • Technical findings paired with management strategies offer actionable insights for land-use planning and ecosystem resilience.
Leaf Area Index of Sharada Khola Watershed

 

Evapotranspiration for Sharadha Khola Watershed
Related space-based solutions
Keywords (for the solution)
Climate Zone (addressed by the solution)
Habitat (addressed by the solution)
Region/Country (the solution was designed for, if any)
Relevant SDGs