MODIS estimates of annual Evapotranspiration of irrigated crops in the Nile delta based on the FAO method.

Application to the Nile river budget.

Simonneaux V.1, Abdrabbo M.A.A.2, Saleh S.M.2, Hassanein M.K.2, Abou-Hadid A.F.3, Chehbouni A.1

1 Centre d'Etudes Spatiales de la Biosphère : CESBIO (UMR : CNES-CNRS-UPS-IRD); 18, avenue Edouard Belin, 31401 Toulouse Cedex 9 – France. vincent.simonneaux@ird.fr Fax+33 5 61 55 85 00

2 The Central Laboratory for Agricultural Climate (CLAC/ARC), Cairo, Egypt 3 Agricultural Research Center (ARC), Director, Cairo, Egypt

Abstract

Egypt is experiencing a water crisis because all resources are used and the Nile is not able to provide for the increasing needs especially for irrigation. In order to better manage the available resources, an accurate quantitative assessment of water fluxes, and especially evapotranspiration, is needed to improve the management of irrigation water. We show here how MODIS low resolution satellite images (250m) were used, jointly with climatic data interpolated from stations, to compute the annual irrigation consumption of the Nile delta based on the FAO-56 method. The results are used to estimate of the Water budget of Egypt and give insights about irrigation efficiency.

Keywords: Egypt, Nile, Irrigation, Evapotranspiration, Remote sensing, MODIS, Water budget

1 INTRODUCTION

The Nile river accounts for more than 95% of all water resources of Egypt, from which at least 85% are used for irrigation1,2. All available water resources are already used. The Nile outflow to the sea, which was high and irregular before the building of the Nasser dam3, is now around 2.5-4x109 m3. This is the minimum necessary for the system to function, and especially to avoid the intrusion of sea water4. Moreover, the total amount of water available for Egypt from the Nasser Lake, fixed a minimum value of 55.5x109 m3, although it may be higher for the wet years upstream, but may also be lower when drought occur. In this context, the study of climatic changes is a very hot concern for Egypt. In addition to this very high sensitivity to natural conditions, some political factors are appearing, as the countries located in the upper part of the watershed want to reconsider the old agreement of water sharing, and Sudan and Ethiopia are mentioning irrigation projects. Thus Egypt today is facing a strong water crisis, as it is not able to provide for the increasing requirements of irrigation and domestic use, correlated to demographic growth, and furthermore the situation may get worst in future years.

Traditional irrigation is by far the main management mode in Egypt. Small farmers take water from the channels with small motor pumps to apply gravity irrigation in the fields. This small irrigation is said to be wasting water, because it is managed empirically. The efficiency of irrigation is said to be about 50%5,1. However, this efficiency is usually estimated locally6, and neglect the fact that (1) excess water applied is necessary for salt leaching, which has been a necessary practice for a long time7,8, and (2) most of drainage water is reused downstream for irrigation, and not only locally6.

Remote Sensing for Agriculture, Ecosystems, and Hydrology XII, edited by Christopher M. U. Neale, Antonino Maltese,Proc. of SPIE Vol. 7824, 78241S · © 2010 SPIE · CCC code: 0277-786X/10/$18 · doi: 10.1117/12.865066

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Indeed, a significant fraction of drainage is re-used sometimes far from the production area, because on the whole actually only a small fraction of this drainage water flows to the sea4. Taking into account this generalized reuse of water, other sources mention an efficiency of 82% at the Nile scale in Egypt9. However these estimates are crude and there is a need for a global assessment of irrigation efficiency in Egypt.

Remote sensing, because it offers the unique possibility to monitor periodically the actual vegetation development on large areas, offers the ability to map water consumption on large scale. In the frame of the SudMed project10, we already demonstrated the use of remote sensing image time series for evapotranspiration (ET) estimates and water budget of irrigation11,12. In these works, ET is obtained using the FAO-56 method13, for which the crop coefficients have been derived from NDVI computed from satellite images. In this study, we used the same method with free medium resolution MODIS image time series to compute the annual irrigation consumption of the Nile delta, and bring an estimate of the overall irrigation efficiency of the Nile water in Egypt.

2 MATERIAL AND METHODS

The Nile Delta is covering about 250 by 150 km of fully irrigated agricultural land. Most of it is managed traditionally by small farmers, with small field sizes, which are moreover narrow shaped due to subdivisions linked with inheritance. The crop pattern exhibits two seasons, winter crops from 01/10 to 15/05 (mainly wheat, sugar beet or clover), then summer crops from 15/05 to 30/09 (mainly maize, rice, cotton or sunflower).

SAMIR is a software tool aimed at spatializing ET at a daily time step based on the FAO method linked with high resolution NDVI time series for deriving crop coefficients14. A soil module is linked to compute soil water budget at the daily step (Runoff is not considered in irrigated areas). The irrigation heights are either input if known, or estimated from the water budget of the crop, following user chosen rules for water management. As actual irrigation is usually not available exhaustively at the plot scale, the plot budget is only a simulation, and may not be used for plot management. However, at the perimeter scale or above, errors compensate and give interesting estimates of consumption and requirements to managers. The main asset of SAMIR is that crop coefficients are linked to actual crop stages observed, and not arbitrary ones used in many studies. This key feature is a very significant advantage in areas where crop development is not always optimal due to water or other technical or human constrains.

Because of the size of the study area, it would be very difficult and costly to monitor using high resolution satellite imagery (ten images required for the delta). Free low resolution MODIS images (250 m resolution) were chosen instead, and a time series of 24 MODIS images was downloaded covering the period from 29/09/2008 to 29/09/2009.

The reference evapotranspiration (ET0 Penman) was obtained from meteorological stations belonging to the CLAC meteorological network distributed over irrigated areas of Egypt. Data was available for 20 stations, among which 11 were covering the delta. The time stretch available ranged from 2 to 4 years depending on stations, in the period 2003-2006.

3 RESULTS

3.1 Pre-processing data for input in SAMIR

Running SAMIR requires basically three types of information: (1) NDVI time series and a link between NDVI and crop coefficients, (2) climatic data, (3) knowledge about soil properties and irrigation practices. Because we are working here on a very broad scale, simplifications will be necessary.

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Regarding crop coefficients, the basic assumption linking NDVI to Kcb, which is specific from a crop type, can’t be done here. As we were using low resolution images, each pixel doesn’t match with a unique crop type, but a mixture of several ones. Thus, we use an equivalent crop coefficient representative of the crop distribution. Moreover, very little information is available in Egypt about crop coefficients15, and even less about relations with NDVI. Thus, we used SAMIR in a simplified way, considering for all MODIS pixels one single NDVI-Kcb relation, issued from previous studies in Morocco for wheat16.

The ET0 data available was scattered, between only one and four years were available depending on the station. Moreover there was no data for the year 2008-2009. Thus, we decided to compute an average daily ET0 value for each station, using available data. The approximation done in using average data instead of the specific images’ years seems quite reasonable since inter annual variability of ET0 at any location is usually lower than spatial variability. The daily ET0 values obtained range from 2 to 10 mm, depending on date and location (Figure 1). As average values are computed using few values, the profiles still look noisy, but on the whole the annual time course is well figured, and spatial heterogeneity is well reproduced. These average annual profiles were used subsequently in SAMIR to interpolate daily fields of ET0 over Delta and Nile valley.

For illustration purpose, the cumulated map of all daily fields was computed (Figure 2). It shows that annual ET0 values range from 1400 to 2600 mm. A very strong gradient appears from north to south, linked to the contrast between the marine and desert influences.

Average ET0 (mm) measured by the 20 CLAC stations

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Figure 1: Average daily ET0 values for the20 CLAC stations.

Rainfall is very scarce and irregular in space and time in Egypt. Little data was available to spatialize rainfall, but a map of Milewski17 shows that the delta receives between 25 and 100 mm annually. Thus, as rainfall is anyhow a minor term in the water balance, a crude hypothesis was made, selecting arbitrary one year available with 61 mm found in the available data, and applied to the whole delta.

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Figure 2: Distribution of annual ET0 over Egypt. Interpolation of CLAC stations data.

Finally, to quantify the irrigation practices, as it was out of the scope of this study to look in details to water management, we chose a global threshold for triggering irrigation and an amount to apply. Despite irrigation requirements are roughly satisfied at the scale of Nile valley and delta, it is recognized that stress appears here and there some years, which is consistent with the global water shortage appearing in Egypt. Thus irrigation was triggered in SAMIR when there was 20% water left in the total field water capacity (FWC), which means that the easily available field capacity (usually around half the total FWC) was already empty and that some stress was already occurring. For each irrigation event, the FWC was just filled.

3.2 Outputs and Egypt water budget

The average time profile for the whole delta clearly shows the two main crop seasons (Figure 3).

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Figure 3: Time profile of MODIS NDVI averaged over the delta ( 01/10/2008 - 30/09/2009).

Although SAMIR produces basically daily maps related to water budget (ET, irrigation, soil water content, etc.), our main concern for this study is the annual water budget. Figure 4 shows the cumulated ET over the delta for the year 2008-2009. The average annual consumption computed only on vegetated areas is 1150 mm, but the map shows marked heterogeneities. Dark areas corresponding to urban areas are clearly visible. Consumption appear to be lower at the fringe of the delta, which is consistent with the fact these are areas more recently reclaimed on the desert, where water is less available and agriculture less intensive. Many orchards on bare soil are also found there, with a limited vegetation fraction cover, which means also less water consumption. The results also show that summer crops consume as much water as the winter ones (575mm each), although their duration is shorter. These results for one specific year need to be validated in looking more closely at this map to identify the underlying vegetation, and also studying other years to assess the inter annual variability of ET.

The SAMIR estimates of ET have been used subsequently to study the global Nile water budget. The first aim of this analysis is to allow an indirect validation of our satellite approach by checking a satisfactory closure of this budget. Another outcome is to give us insights about potential deep fluxes between the delta aquifer and the Mediterranean Sea. Figure 5 shows the outline of a simplified water budget of Egypt, along with the associated equation. We make the hypothesis that on the whole, the aquifer system of Egypt is confined, except the possible deep fluxes in the delta. Thus, the inputs of the system are the Nasser dam release, and the rainfall. The outputs are ET, urban and industrial consumption, Nile flow to the sea, and the potential deep fluxes. As a lot of internal re-use of drainage waters occurs inside the delta, the actual net losses of the system, i.e. water flowing to the sea, can be estimated as these deep fluxes.

The area of crops obtained from the MODIS images is 2.6x106 ha for the delta, for which the total amount of ET was 30x109 m3. We achieved a quick extrapolation to the consumption of the whole Egypt, based on the relative surfaces of vegetation in the delta and in the valley. The surface of irrigation in the valley was obtained by simply applying a threshold on NDVI for a single MODIS composite image covering whole Egypt. We obtained a total area of 3.9x106 ha, including delta, valley, and oasis. Thus a crude extrapolation of SAMIR ET estimates gives 44x109 m3 of annual consumption. The Nasser dam is assumed to release annually 55.5x109 m3 of water. The rainfall on all irrigated areas is 2.5x109 m3. The outflow to the sea is assumed to be about 3x109 m3. For urban and industrial uses1, values found are respectively 4.5x109 m3 and 7.5x109 m3, but these estimates neglect the sewage going back to Nile, which is very significant. Finally, the direct evaporation of water in the Nile River and in irrigation channels has not been accounted for. However, if we assume that 1% of irrigated areas are open water surfaces, and that direct evaporation is not more than two times the ET of vegetation, then evaporation losses are less than 2% of the irrigation consumption. This quick

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approximation is quite uncertain, but only shows that evaporation from channels may not be a very significant term in the budget. Thus, solving the budget equation, we find that the sum of net urban and industrial use and deep fluxes is 11x109 m3. Considering that most of the 12x109 m3 of urban and industrial water is released as sewage, there is not much room for deep losses to the sea…

Figure 4: Annual evapotranspiration of irrigation (mm), period 10/2008 – 09/2009

Figure 5: Simplified water budget of Egypt

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This budget includes several approximations and is to refine, but our purpose was here to provide with the magnitude of the various terms. In this regard, it confirms that ET issued from remotely sensed crop coefficient gives robust and realistic results. It also shows that most of the Nile water is consumed by ET and that drainage loss to the sea, although not accurately estimated, may not be that huge, as it is often stated. The low irrigation efficiency observed at the plot level cannot be generalized at higher scales.

4 CONCLUSION AND PERSPECTIVES

This first approach showed the potentialities of low resolution satellite data for ET monitoring. The computation is based on a rough hypothesis of a single NDVI-Kcb relation, and requires validation. However, the global water consumption estimated for irrigation is quite consistent with the known water budget of Egypt.

Many parameters have to be improved (land cover, irrigation practices, and climatic data):

- Analysis of land cover data available from previous projects will be very useful to refine the estimates of lumped Kcb used with MODIS.

- High resolution satellite time series may be used to disaggregate lumped MODIS estimates in single crop consumptions.

- A more detailed validation of ET and SAMIR outputs is required using ground measurements, irrigation ground monitoring on smaller spatial units, and at finer time scales.

- Efforts have to be made to refine the terms of Egypt water budget.

5 REFERENCES

[1] PNUE, PAM, Plan Bleu, “L'eau des Méditerranéens : situation et perspectives”, 158, PNUE/PAM, Athènes, (2004).

[2] Hamza, W., “Estuary of the Nile”, In Wangersky P. (ed): Estuaries Handbook of Environmental Chemistry 5H, Springer Verlage, Heidelberg, 149–173 (2006).

[3] Sharaf el Din, S.H., “Effect of the Aswan High Dam on the Nile Flood and on the Estuarine and Coastal Circulation Pattern Along the Mediterranean Egyptian Coast”, Limnology and Oceanography 22(2), 194-207 (1977).

[4] Hamza, W., “The Nile Delta”, In H.J. Dumont (ed.): The Nile: Origin Environments Limnology and Human Use, Springer Science Business Media B.V., (2009).

[5] Bethemont, J., “Le Nil, l’Egypte et les autres”, Vertigo – La revue en sciences de l'environnement, 4(3), (2003).

[6] El Shorbagy, W., “Impact Assessment of an Irrigation Improvement Project in Egypt”, Water Resources Management 14, 229–246 (2000).

[7] Ruf, T., “Histoire hydraulique et agricole et lutte contre la salinisation dans le delta du Nil”, Sécheresse 4(6), 304-317 (1995).

[8] Kotb, T.H.S., Watanabe, T., Ogino, Y., Tanji, K.K., “ Soil salinization in the Nile Delta and related policy issues in Egypt”, Agricultural Water Management 43, 239-261 (2000).

[9] Waterwiki, “Irrigation in Egypt”, http://waterwiki.net/index.php/Irrigation_in_Egypt (2010)

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[10] Chehbouni, A, Escadafal, R., Duchemin, B., Boulet, G., Simonneaux, V., Dedieu, G., Mougenot, B., Khabba, S., Kharrou, H., Maisongrande, Ph., Merlin, O., Chaponnière, A., Ezzahar, J., Er-Raki, S., Hoedjes, J., Hadria, R., Abourida, A., Cheggour, A., Raibi, F., Boudhar, A., Benhadj, I., Hanich, L., Benkaddour, A., Guemouria, N., Lahrouni, A., Olioso, A., Jacob, F., Williams, D. G., Sobrino, J. A., “An integrated modelling and remote sensing approach for hydrological study in arid and semi-arid regions: the SUDMED Program”, International Journal of Remote Sensing 29(17-18), 5161-5181 (2008).

[11] Duchemin, B., Er-Raki, S., Gentine, P., Maisongrande, C., Coret, L., Boulet, G., Rodriguez, J.C., Simonneaux, V., Chehbouni, A., Dedieu, G., Guemouria, N., “Estimating cereal evapotranspiration using a simple model driven by satellite data”, Proceedings International IEEE Geoscience and Remote Sensing Symposium (IGARSS’03), Toulouse, France, July, 21-25, vol.4, 2762-2764 (2003).

[12] Simonneaux, V., Duchemin, B., Helson, D., Er-Raki, S., Olioso, A., Chehbouni, A., ”Using high resolution image time series for crop classification and evapotranspiration estimate over an irrigated area in south Morocco”, International Journal of remote Sensing 29(1), 95-116 (2008).

[13] Allen, R.G., Pereira, L.S., Raes, D., Smith, M., “Crop evapotranspiration: guidelines for computing crop water requirements”, FAO Irrigation and Drainage Paper 56, Rome, 300 pp. (1998).

[14] Simonneaux, V., Lepage, M., Helson, D., Metral, J., Thomas, S, Duchemin, B., Cherkaoui, M., Kharrou, H., Berjami, B., Chehbouni, A., “Estimation spatialisée de l’Evapotranspiration des cultures irriguées par télédétection. Application à la gestion de l’Irrigation dans la plaine du Haouz (Marrakech, Maroc)”, Sécheresse 20(1), 123-130 (2009).

[15] Abou El-Magd, I.H., “Estimation of the spatial distribution of crop coefficient (Kc) from Landsat satellite imagery”, Egypt. J. Remote Sensing and Space Sci. 12, 43-54 (2009).

[16] Duchemin, B., Hadria, R., Er-Raki, S., Boulet, G., Maisongrande, P., Chehbouni, A., Escadafal, R., Ezzahar, J., Hoedjes, J.C.B., Kharrou, M.H., Khabba, S., Mougenot, B., Olioso, A., Rodriguez, J.C., Simonneaux, V., “Monitoring wheat phenology and irrigation in Center of Morocco: on the use of relationship between evapotranspiration, crops coefficients, leaf area index and remotely-sensed vegetation indices”, Agricultural Water Management 79, 1–27 (2006).

[17] Milewski, A., Sultan, M., Yan, E., Becker, R., Abdeldayem, A., Soliman, F., Abdel Gelil, K., “A remote sensing solution for estimating runoff and recharge in arid environments”, Journal of Hydrology 373(1-2), 1-14 (2009).

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