Water Balance: Actual Evapotranspiration & Precipitation

1. Status

Results for SAI and SASS basins are generated and discussed.

2. Product Specifications

Size:	SAI (around 500.000km²) SASS (around 1 Mio. km²)
Scale:	basin wide
Minimum accuracy:	--
Area:	SAI and SASS
Benefits / Comments:	Precipitation and evapotranspiration are the main parameter in assessment of overall water balance
EO Data:	Meteosat 7, meteorological stataion data
For more specifications refer to:	R2_1: Technical Specification and Service Cases Description

3. Illustrations

Results are compiled based on METEOSAT First Generation data.

SAI
Basin wide: Annual rainfall for 2005

Basin wide: Annual actual evapotranspiration for 2005

Basin wide: Water balance for 2005

4. Maps

	Type/Size	Date
3 Maps - Water Balance - SAI	ZIP/893 kB	02.07.2007
3 Maps - Water Balance - SASS	ZIP/2487 kB	02.07.2007

5. Poster

	Type/Size	Date
Large Scale Water Balance Assessment with METEOSAT	PDF/3884 kB	25.06.2007

6. Explanation

Precipitation: Precipitation is a central component of the water balance. For large scale, basin wide precipitation assessment, meteorological satellite data are of use for improved spatial interpolation of rare rainfall station data.

Evapotranspiration: Actual evapotranspiration (ETa) cannot be measured directly. For a basin wide - spatial mapping of the actual evapotranspiration only a combined application of energy balance modelling and meteorological satellite data analyses can be used.

Results for SAI:

The annual precipitation, actual evapotranspiration and water balance were calculated during the SAI workshop. This serves as basic input information for aquifer management. For calibration, validation and quality control meteorological station data from WMO GTS network were used.

The spatial distribution of precipitation for 36 ten-days intervals in 2005 in low spatial resolution (around 5 km) were obtained from METEOSAT 7. Maps of basin-wide actual evapotranspiration were derived from METEOSAT 7 analyses with the same temporal coverage and spatial resolution.

The resulting water balance was calculated as difference between rainfall and actual evapotranspiration. Positive values indicate that not all rainfall water was used by vegetation, but has contributed to surface runoff or infiltrated to the aquifer. Large negative values can indicate that the region was irrigated, since the rainfall alone is not sufficient for the observed actual evapotranspiration. The map of the water balance is an important input for aquifer management highlighting the regions with water surplus and deficit regions.

Large scale water balance assessment using METEOSAT has also limitations. For example the extended negative water balance at the Northern border of the SAI aquifer (see map above) , which is a result of a modelled actual evapotranspiration being higher than rainfall, is unrealistic. With annual rainfall values of less than 100mm and actual evapotranspiration sums of 100 mm to 200 mm, the accuracy of the METEOSAT based approach appears here to be not sufficient for a proper water balance calculation.

The precipitation and ETa products were integrated into the available software setting at Agrhymet, CERMES and DRE (Niger). The approach was demonstrated using first generation Meteosat data, but can be extended also to MSG data, as currently received by Agrhymet. Recommendations were derived during the SAI workshop - please refer to the corresponding mission report for further details.

7. User Assessment - Validation

2006: DRE - CERMES - ARC, Niger
Results were jointy prepared and analysed during a workshop with participants from DRE, CERMES and Agrhymet.

To make full use of the potential of EO data in aquifer management the full assimilation of the EO derived information into agro-meteorlogical models is nessesary. Improved water balance assessments and yield estimations would results from such an approach. First ideas and recommendations for this necessary future modelling infrastructure were given.

March 2007: DRE - Niger
+ estimations tres théoriques, besoin d’être calés avec les faits observés dans les stations synoptiques
+ besoin d’une série d’observation plus longue (20/50 ans)
+ la carte des précipitations traduit bien les grandes zones climatiques
- le modèle doit être corrigé, amélioré, afin d’éviter de présenter de fausses zones de déficit hydrique

June 2007: DNH - Mali
évaluation globale: moyenne
+ : auraient souhaité participer à l’élaboration de ces produits
+ (précipitations, ETP, bilan hydrique) qui peuvent indiquer les zones
+ potentielles de recharge des aquifères d’extension régionale (CI, CT)
+ : ces produits donnent une zonation climatique qui croisée avec les
+ données biomasse peut être très utile
- : une seule année ne permet pas une analyse sérieuse, et l’absence de visite de terrain n’a pas permis de recueillir le point de vue des agents DNH sur place Atelier