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Abstract

Date is an important cash crop in the eastern part of Yemen. Water management for date plantations in arid areas has become an important aspect for food production. Point measurement of water needed by one Date plantation cannot provide accurate esti-mate for all the orchards in a county: the point measurements are costly and unpractical. A new approach is suggested to estimate water requirement by date at a county scale. The proposed tech-nique used LANDSAT-ETM data and M-SEBAL (Modified - Surface Energy Balance Algorithm for Land) to estimate evapotranspiration over date plantation in wadi Hadhramout- basin central of Yemen. The M-SEBAL model estimates evapotranspiration (ET) using the energy balance equations, for which the surface temperature and reflectance data from ETM image data and meteorological data from a local weather station. The average relative error between estimated and observed ET is 10.6%, and the average absolute error is 0.35 mm/day. This proposed technique has the potential to provide guidelines for various users, including government agen-cies on how to evaluate current water-usage schemes. Introduction

The water use of Date, an important crop in eastern Yemen, is about 1.6 m to 2.4 m of irrigation water per year (AREA, 2005). The main objective of this study is to study the feasibility of using M-SEBAL to estimate the spatial distribution of date water use over an arid region with non-homogeneous surface conditions in Wadi Hadramout- Yemen with different processing levels, LANDSAT 7 ETM, for the growing season of 2006.

Study area

Wadi Hadramout is located in the central part of Yemen. It is one of the most important agricultural areas in the eastern and desert region (see figure 1). The study area covers approximately 2000 km2, between Heinen (UTM 921000 E, 1754000 N) and Qasam (930400 E, 1784000 N).

Dr. Ayoub Ahmed Almhab Christiaan van der Tol, AbdulFattah Alghorbani

Department of Water Resources, Faculty of Geo-Information Science and Earth Observation,

University of Twente, Enschede, The Netherlands, Phon (0617070573),

mail: almhab@itc.nl (ayoub.almhab@gmail.com)

Regional evapotranspiration of Date using Landsat and M-SEBAL

Start

Input data: satellite data, local weather data

Data preprocessing: geometric

Generation of DEM: slop/aspect and theta

Calculation of surface reflectance

Generating model parameters

Determination of radiations

Calculation of ET: daily, monthly

End

The M-SEBAL model was implemented using Model Builder in the ERDAS Imagine 8.5 image processing package. All the model pa-rameters were programmed into the Model Builder and the values are computed automatically based on the input data.

Methods

SEBAL is an algorithm based on the energy balance at the Earth’s surface where the values of the requisite parameters are obtained from remotely sensed data. The method uses the energy budget equation to calculate the instantaneous latent heat loss Rn – G – H for each pixel at the time of the satellite over flight, where Rn is the net solar radiation, G is the soil heat flux into the soil, and H is the sensible heat flux into the air (all in W m-2). The instantane-ous evapotranspiration ETins at the time of the satellite overflight is then related to the instantaneous latent heat loss via the latent heat of vaporization, l, so that :

l(ET)= Rn – G – H Eq. 1

The earliest applications of the model were to rather flat, well-watered terrain. Since our study area is mountainous and arid, or at best semi-arid, we have developed M-SEBAL by introducing some changes into the existing SEBAL model to allow for the inclusion of the effects of terrain, mountains and deserts into the calculations of surface radiation. The four steps in M-SEBAL are shown in Figure 2.

Fig. 1 Location of the Wadi Hadramout in central Yemen, (true color image of Landsat 7, image dated 6/01/06).

Results

M-SEBAL is sensitive to estimate some parameters such as vegeta-tion index, surface temperature and evapotranspiration. This can be seen from the values of surface parameters derived from satel-lite image data.Area which high vegetation induces has a low surface tempera-ture. Area with low vegetation indices has high surface tempera-ture. This seen especially in the part where there are near middle of the valley which represent the high vegetation density and the part far from the valley area has the opposite situation. Figure 3 shows that daily ET increases from bare soil to where vegetation indices are high. Figure 3 shows comparison between ET derived from LANDSAT 7 ETM using modified SEBAL and ET calculation using conventional ET models using data from meteorological data in Wadi Hadramout.

Conclusions

The M-SEBAL model is capable of calculating the spatial Date daily water use (ET) with resolution of 90 m × 90 m. The simulated is accurate compared with measurement under both stressed and no stressed conditions. This study demonstrated that some surface parameters such surface albedo, vegetation index, surface temperature can be accurately derived from satellite image data. In this case surface parameters have been derived from LANDSAT 7 ETM images, with the help of ground truth weather data values of derived surface parameters for validation .

Acknowledgement

This research was supported by the Erasmus Mundus External Cooperation Window Program of the European Union, the satel-lite image data Landsat TM from the from NASA website. Climate data were acquired from the NWRA, Yemen.

Fig. 2. Flowchart for the estimation of daily ET.

Figure 3 . Estimated surface albedo, vegetation index NDVI, sur-face temperature LST (K), daily evapotranspiration ET24 derived from LANDSAT 7 ETM for wadi, Hadramout, Yemen.

Figure 4. Scatter plot of ET as function of differential temperature dT.

Figure 5. simulated ET from the linear relationship between A,B and ET, versus ET calculated from ground-measured meteoro-logical data for 4 dates.

Figure 6 ET of Date simulated and observated.