<<SEBAL presented graphically (comic)>>

Evapotranspiration, the sum of evaporation and plant transpiration, is an important part of the hydrological cycle. Evaporation is the movement (vaporization) of water to the air from the soil, water bodies and canopy interception. Transpiration is the loss of water as vapour through stomata in the plants’ leaves. Evapotranspiration is an indicator of how much water vegetation needs for healthy growth and productivity.

Evapotranspiration is not only an important part of the hydrological cycle. Because evapotranspiration uses energy it is also a component of the energy balance. Each water molecule that becomes water vapour takes a parcel of heat with it. The energy that is required for the evapotranspiration is provided by the sun.

The earth is constantly exposed to energy from the sun. Solar energy drives many processes on the earth’s surface. Figure 1 shows the different components of the earth’s energy balance.

Most obviously solar energy causes changes in temperature, for example the heating up of air and soil. Hence, temperature differences produce winds.

A very small part of the sun’s energy is used by plants for photosynthesis. However, this component is so small in comparison with the other components it is usually ignored in the calculation of the energy balance.

Part of the solar energy is absorbed in the hydrological cycle by driving evapotranspiration. The amount of solar energy used for evapotranspiration depends on the amount of water which is available and also on the meteorological conditions. For example, when the relative humidity and wind speed are high, the evapotranspiration process needs less energy.

Not all incoming energy is converted to other types of energy; a part is reflected back into space. How much energy is reflected back depends among others on the earth surface’s reflectivity (albedo). Bright surfaces such as desert reflect more energy than dark surfaces (i.e. forest, water).

The energy balance for irrigated land	The energy balance for rainfed land

When enough water is available, for example in irrigated land with a dense and healthy vegetation cover, the evapotranspiration process consumes a significant part of the solar energy. Little energy remains for the heating up of air and soil, and as a result soil and air temperature are lower.

The Surface Energy Balance Algorithm for Land (SEBAL) uses the energy balance to estimate some aspects of the hydrological cycle. SEBAL maps evapotranspiration, biomass growth, water deficit and soil moisture. The basis of SEBAL is the energy balance: the energy driving the hydrological cycle is equal to the incoming energy minus

1. the energy going to heating of the soil and air, and
2. the energy reflected back to space.

The energy balance can be quantified from satellite data. Land surface characteristics such as surface albedo, leaf area index, the vegetation index and surface temperature are derived from satellite imagery. In addition to satellite images, the SEBAL model requires some meteorological data, such as wind speed, humidity, solar radiation and air temperature. MeteoLook can be applied to extrapolate meteorological point measurements to spatial representations of wind speed, humidity and air temperature. Since the SEBAL model uses the energy balance, and not the water balance, no data on land cover, soil type or hydrological conditions is required.

The energy balance applies to all scales: from global level to river basin to region to farm and field. Satellite images come in a wide variety of spatial and temporal resolutions. Evapotranspiration and biomass production are key indicators for water management and irrigation performance. SEBAL applies the energy balance in combination with satellite imagery to map these key indicators in time and space for days, weeks or years.