Satellite-based evapotranspiration estimation of the green scheme irrigation projects in Namibia

Introduction

Serious water shortages are developing in many parts of Sub-Saharan African countries and water for agriculture is becoming increasingly scarce as a result of growing water demands from different sectors [1]. Rainfed agriculture is said to return relatively low yields, especially in the dry arid and semi-arid regions of Sub-Saharan Africa such as Namibia. Therefore, the Namibian government has embarked on development of irrigation schemes as the key to achieving sustainable water management. However, designing rational irrigation systems requires a good knowledge of the hydrological cycle, which is mainly controlled by evapotranspiration (ET), second only to rainfall [2]. This study promotes monitoring and assessment of ET for sustainable irrigation practices using different ET estimation and assessment methods.

Methods

This paper focuses on the quantification of ET in the Okavango-Omatako water basins in Namibia, using different methods including the models SEBAL, FAO-56 PM and Samani-Hargreaves. Epan ETo (Class A pan measurements) were used for validation. Meteorological data, such as air temperature, wind speed, solar radiation, relative humidity and precipitation were acquired from Namibia Meteorological Service and SASSCAL stations. Cloud free Landsat TM satellite data for SEBAL model calculations were acquired from NASA via http://earthexplore.usgs.gov.

Results and conclusions

Statistical analysis showed that FAO-56 PM and Samani-Hargreaves ET correlated very well with Epan ETo, ranging from 0.85 to 0.97, whereas SEBAL correlated poorly, with correlation values of 0.41 and 0.43. Overall, all the methods, including SEBAL, portrayed reasonably good time series visual relationships with Epan measurements.

Understanding the relationship between various methods is fundamental in choosing the most appropriate method with the highest accuracy for ET estimation. In general the results demonstrate a range of differences which were positive and negative. There is a potential to establish a conclusive relationship, both statistically and visually, between Samani-Hargreaves, FAO-56 PM, and Epan ETo. On the other hand various studies have shown a strong positive relationship between SEBAL model values and Epan.

Please note these Proceedings have received limited or no review from supervisors and IIASA program directors, and the views and results expressed therein do not necessarily represent IIASA, its National Member Organizations, or other organizations supporting the work.   

Last edited: 01 February 2016