Agricultural Water Management 45 (2000) 203±214

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1 Agricultural Water Management 45 (2000) 203±214 Estimation of crop water requirements in arid region using Penman±Monteith equation with derived crop coef cients: a case study on Acala cotton in Sudan Gezira irrigated scheme A.W. Abdelhadi *, Takeshi Hata, Haruya Tanakamaru, Akio Tada, M.A. Tariq Graduate School of Science and Technology, Department of Regional Environment, Kobe University, Rokkodai, Nada-ku, Kobe , Japan Accepted 10 September 1999 Abstract The recommended Penman±Monteith reference crop evapotranspiration (ET 0 ) with derived crop coef cients (K c ) from the phenomenological stages of Acala cotton is used to estimate the crop water requirements (CWRs) of Acala cotton in the Gezira area of Sudan. The published basal crop factors of Acala cotton were used with Penman±Monteith equation as well to estimate ET. The results were compared with the current practice that uses Penman evaporation (E 0 ) from free water surface and crop factors (K f ) derived by Farbrother [Farbrother, H.G., Irrigation practices on Gezira clay-rates and intervals. Gezira miscellaneous paper no. 94. Gezira Research Station, Wad Medani, Sudan] and still in use in Sudan. The two methods were compared with the actual ET of Acala cotton measured by Fadl [Fadl, O.A., Water use of Acala cotton. Annual report 1978±1979. Gezira Research Station, Wad Medani, Sudan, pp. 143±147]. Penman±Monteith equation was found to be better than Farbrother method in terms of the total predicted CWR, coef cient of determination (r 2 ), the slope of the linear regression line and the standard error of estimate with both basal and derived (K c ) values. The trends of weather examined for the period 1966±1993 showed an increasing ET 0 during the rainy season due to the recent drought conditions that prevailed in the region. Care must be taken when predicting CWR during such period. # 2000 Elsevier Science B.V. All rights reserved. Keywords: Penman evaporation; Penman±Monteith; Reference crop evapotranspiration; Crop water requirements; Crop factors; Crop coef cients; Acala cotton; Gezira scheme * Corresponding author. address: hadi@ans.ans.kobe-u.ac.jp (A.W. Abdelhadi) /00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved. PII: S (99)

2 204 A.W. Abdelhadi et al. / Agricultural Water Management 45 (2000) 203± Introduction Sudan (Africa's largest country) is overwhelmingly dependent on agriculture, where more than one quarter of its population are involved in agricultural activities. Most of the country's foreign exchange earnings come from the irrigated sector. Cotton, wheat, groundnut, sorghum, vegetables and forage crops are produced under full or supplementary irrigation. Sudan possesses the third largest irrigated area in Africa with 1.3 million hectare at present irrigated from the Blue Nile. The Blue Nile flow pattern is marked by a pronounced seasonality. Its normal annual flow (1912±1989) is 49.2 billion cubic meters. About 71% of this flow occur during the short flood season (July± September), while this figure drops to 4% during the driest period (January±April). The lack of proper storage facilities renders the utilization of the country's water share uncompleted. The combined capacities of the two reservoirs of the Blue Nile (Roseires and Sennar) fall short of half the irrigation requirements during the dry season. Moreover, the capacities of the reservoirs are decreasing due to siltation. Prediction of crop water requirement (CWR) is of vital importance in water resources management and planning in Sudan. Since the intensification of the cropping pattern of the Gezira scheme ( ha), the empirical method of water indenting that assumes the requirements of all crops at 71.4 m 3 ha 1 per day was proved to be inappropriate by Farbrother (1984). A more scientific method was introduced in the early 1970s by Farbrother (1970, 1979) and Adam and Farbrother (1977). The method is based on the calculation of water needed by plants to satisfy evapotranspiration losses measured from soil moisture depletion via daily gravimetric sampling. The sampling was done on 10± 20 cm depth intervals up to 1 m. The calculated ET values were related to the original Penman evaporation from free water surface via a crop factor (K f ). On the other hand, Doorenbos and Pruitt (1977) presented a similar method for the prediction of CWR. Both methods were based on Penman evaporation equation. Farbrother used the original Penman (1948) evaporation equation with the wind function suggested by Penman (1956) which was never calibrated for the Gezira conditions. Doorenbos and Pruitt (1977) method used a slightly modified version of the equation with a revised wind function where the evapotranspiration (ET 0 ) from reference short grass was determined. Allen et al. (1994) argued that Doorenbos and Pruitt (1977) method tends to overestimate ET and instead he presented another equation based on Penman±Monteith that determines ET 0 of a hypothetical grass. The Penman±Monteith equation with its new definition of ET 0 is recommended by FAO experts as the standard method of CWR calculation. This will solve the problem of estimating crop ET with respect to different kinds of reference grass (warm or cool-season grass) as shown by Fadl (1978) and discussed by Ahmed and Ahmed (1989). The only remaining difficulty would be the absence of calculated K c values for most of the arable crops in areas where the Penman±Monteith was not adopted. The determination of K c is of more importance for large irrigated projects such as the Gezira scheme that is considered as one of the largest of its kind under a single administration body. The Gezira scheme is shown together with the major irrigation schemes on the Blue Nile in Fig. 1. The climate of the region is arid and continental with low average annual precipitation (472 mm at Sennar dam and 160 mm near Khartoum). The rainy season is

3 A.W. Abdelhadi et al. / Agricultural Water Management 45 (2000) 203± Fig. 1. Major irrigation schemes of the Blue Nile dominated by the Gezira scheme. short (July±September) with moderate temperature and high humidity. The summer (April±June) is hot and the winter (November±February) is dry and cool. The rest of the period is transitional. Cotton is the main crop grown in Gezira representing one of the most important cash crops for the country. The seasonal amount of total water releases from Sennar dam to Gezira scheme is about ± m 3. This means an error of 10% in the calculation of CWR would be very large (about the current capacity of Sennar dam). The prediction of CWR represents an important tool for pre-season planning of the year's hydraulic schedule and water indenting in large irrigated schemes in Sudan. Adam (1984) calibrated the wind function of the original Penman evaporation under the Gezira condition. The new wind function obtained would increase Penman evaporation by about

4 206 A.W. Abdelhadi et al. / Agricultural Water Management 45 (2000) 203±214 27%. This leads to the supposition that Farbrother crop factors were thought to be high. Never the less the predicted CWR would not be affected as long as the original Penman evaporation is used to predict the CWR. The objective of this paper is to examine the performance of the recommended method under arid climate and heavy cracking vertisol represented by the Gezira area compared with the current method of Farbrother. The lack of actually measured K c values also provides an opportunity to examine the recommended method with derived K c values compared with the published ones by Ahmed and Ahmed (1989) citing Doorenbos and Kassam (1979). Furthermore, the trends of weather changes with respect to Penman±Monteith monthly ET 0 values for the period 1966±1993 were studied. 2. Methodology Farbrother method defined CWR as the amount of water that is equal to the maximum crop water use (CWU) when soil moisture is adequate and good husbandry practices are followed. A crop factor (K f ) was defined as the ratio between CWU and Penman evaporation (E 0 ). CWR is predicted by multiplying the crop factor during the specified period by the relevant E 0 normalized means obtained from the Gezira Meteorological Station (GMS). Crop factors of the main crops grown in the Gezira region were published by Adam and Farbrother (1977). Penman equation used in Sudan by Farbrother takes the form E 0 ˆ DR n g e a e d f u ; (1) D g where E 0 is the Penman evaporation (mm per day) from open water, D the slope of the saturation vapor pressure (kpa 8C 1 ), R n the net solar radiation (mm per day), g the psychrometric constant (kpa 8C 1 ), e a the saturation vapor pressure at mean temperature (kpa), e d the mean actual vapor pressure (kpa), and f(u) is the wind function suggested by Penman (1956), where u is the wind speed (m s 1 ) at 2 m height. For the calculation of Penman±Monteith ET 0 monthly values, the actual mean monthly relevant weather data were used in the computer program CropWat 4 Windows Version 4.00 Beta of the Food and Agriculture Organization of the United Nations (FAO). The same program was used to calculate the mean monthly ET 0 values for the period 1966±1993 to study the trends of ET 0. The climatological normals of the weather data obtained from the GMS for the period 1961±1990 were also used to calculate ET 0 according to Penman±Monteith equation through the CropWat program. These normalized weather data are currently in use for the prediction of CWR in Sudan. However, for the comparison of the two methods with the actual ET values the actual weather data were used. 3. Determination of K c for Acala cotton The guideline for the calculation of the crop coefficient presented by Doorenbos and Pruitt (1977) was followed to calculate the crop coefficients (K c ) of Acala cotton varieties

5 A.W. Abdelhadi et al. / Agricultural Water Management 45 (2000) 203± Table 1 Acala cotton development stages used for (K c ) derivation (adopted) compared with that obtained from the graph of 10-day mean Farbrother crop factors (K f ) Stage Number of days per stage Adopted Farbrother Initial Crop development Mid-season Late-season Total of Sudan. Acala cotton in the Gezira scheme is usually planted in August. The stages of growth development used in this study were approximated according to the phenomenological development of the crop obtained from the Gezira Research Station. Generally the K c or K f curve reflects an initial stage with low values and then a rising limb during increased growth and a peak where the crop attains maximum cover and growth followed by a decreasing limb when leaves start shedding at the end of the growth cycle. Doorenbos and Pruitt (1977) divided the K c curve into four stages: initial, crop development, mid-season and late-season stages. The change in the slope of the curve reflects a change in the stage. Table 1 shows the length of each stage used in this study as compared with the ones obtained in a similar way from the graph of Farbrother's 10-day mean K f values. According to Doorenbos and Pruitt (1977) the initial K c for two weeks irrigation interval would be between 0.2 and 0.3. The value of 0.25 was taken as the initial K c and the curve was drawn with mid and late-season values of 1.2 and 0.65, respectively. From here on the K c values obtained by this method will be referred to as the derived K c. On the other hand, Ahmed and Ahmed (1989) citing Doorenbos and Kassam (1979) presented K c values of Acala cotton termed the basal K c values. The resulting 10-day means of the derived K c are shown together with the 10-day means of K f and basal K c of Acala cotton in Fig. 2. It is worth mentioning that Acala varieties take about 200 days from sowing to final picking. As irrigation is usually stopped to induce polls ripening far before the final picking the period of 5 months was used for the derivation of the crop coefficients. Similar period was covered by the crop factors developed by Farbrother. Mean monthly values of crop factors were derived graphically from Farbrother's 10-day published crop factors of Acala cotton. The 10-day mean crop coefficients (derived K c and basal K c ) and crop factors were used to calculate Acala cotton CWR using the following equations: ET p ˆ K f E 0 ; (2) ET p ˆ K c ET 0 ; (3) where ET p is the predicted CWR in mm per day, E 0 and ET 0 are the monthly or 10-day mean Penman evaporation and Penman-Monteith reference crop evapotranspiration in mm per day, respectively. It is important to mention here that both the derived and the basal K c values were used in Eq. (3).

6 208 A.W. Abdelhadi et al. / Agricultural Water Management 45 (2000) 203±214 Fig day mean derived and basal K c and K f of Acala cotton. Fadl (1987) measured the actual ET of Acala cotton in the Gezira Research Station from daily soil moisture depletion readings using a Troxler's neutron probe at 10 cm depth intervals up to 160 cm. He published the results in 10-day periods together with the 10-day means of E 0 values during the experiments. His values are referred to here as the actual ET of Acala cotton. The experiments of Farbrother and Fadl were carried out in the Gezira Research Station that shares the same plot orientation and water distribution system of the Gezira scheme. The soil is known as the Gezira clay which is part of the central clay plain that covers about 25 million hectares of the flood plains of the Blue and White Niles. It is a heavy impermeable montomorrilitic vertisol with minor variations in physical and chemical characteristics. There are no deep drainage losses in this soil as reported by many researchers, e.g. Adam and Farbrother (1977), Farbrother (1984), Ibrahim (1984), Ahmed and Ahmed (1989), Elawad (1991) and Ibrahim et al. (1999). The technique of using the neutron probe for water relation studies has gained popularity in the Gezira area since 1971 due to the behavior of the cracking clay that renders other methods impracticable or cumbersome. More details of its application and sampling techniques were given recently by Ibrahim et al. (1999). The effect of the recent weather trends was studied by plotting the 10-year moving averages of the mean monthly ET 0 values for the period 1966± Results and discussion Fig. 3 shows the predicted mean monthly values of Acala cotton using the two methods and the actual ET. It is important to mention here that the actual 10-day mean E 0 and

7 A.W. Abdelhadi et al. / Agricultural Water Management 45 (2000) 203± Fig. 3. Mean monthly predicted and actual CWR of Acala cotton. mean monthly ET 0 values during the time of the experiment were used. Fig. 3 shows that Penman±Monteith ET 0 combined with the derived K c and the basal K c values were more close to the actual values during the initial and development stages when compared with the E 0 combined with Farbrother K f values. However, both methods underestimated the Acala cotton peak ET in November and overestimated the actual requirements at the last stage. Fig. 4 shows the predicted versus the actual ET values on a 10-day basis. Farbrother method underestimated the peak CWR of Acala cotton on November first 10- day period by 0.9%, while the recommended method underestimation were 7.9% and 11.5% when the basal and derived K c values were used, respectively. Farbrother method clearly overestimated Acala cotton CWR during the initial and the development stages, coincided well during two 10-day periods during the actual ET peak and underestimated the decreasing limb before it overestimated the last period. The ranking of the two methods was done under four methods: two were obtained from linear regression of the actual values (Y) on the predicted ones (X). These are the slope and the coefficient of determination (r 2 ). The other two were the total estimated CWR as percent from the actual value and the standard error of estimate (SEE). The result is shown in Table 2. Using the ranking method, Penman±Monteith (with both derived and basal K c values) was better than Farbrother method in all the criteria. This means that accurate estimates of ET can be obtained using Penman±Monteith ET 0 combined with derived or basal K c values. However, the use of basal K c values resulted in higher coefficient of determination and smaller SEE compared with results obtained from derived K c values, while the use of the derived K c values was ranked first in terms of the

8 210 A.W. Abdelhadi et al. / Agricultural Water Management 45 (2000) 203±214 Fig day mean predicted and actual CWR of Acala cotton. slope and the total predicted CWR. This means that the phenomenological stages of the crop can be used successfully to estimate K c values under arid conditions. It is of vital importance for regions where the Penman±Monteith method was not adopted and the determination of field-measured K c values is expensive and time consuming. The total CWR predicted by Farbrother method was 856 mm, a figure close to the 848 mm obtained from CWR tables by Farbrother for Acala cotton under research conditions. The predicted total CWR of Acala cotton by the two methods are compared with the actual one in Fig. 5. Generally, the recommended method with basal and derived K c overestimated the total ET of Acala cotton by about 3.4% and 2.5%, respectively, compared with 20% by Farbrother method. Table 2 Rating results for the two methods de ned by Eqs. (2) and (3) Method SEE (mm per day) r 2 Slope (mm per day) Total CWR as percent of actual ET (%) K f E Basal K c ET Derived K c ET

9 A.W. Abdelhadi et al. / Agricultural Water Management 45 (2000) 203± Fig. 5. Total predicted and actual CWR of Acala cotton. Examination of the 10-year moving averages of the monthly ET 0 values for the period 1966±1993 revealed an increasing trend for the months July, August and September as shown in Fig. 6. No clear trend was found for the other months. Fig. 6 also shows the normalized means (1961±1990) of the mentioned months. The normalized means are in Fig year moving average of mean monthly ET 0 for July, August and September (1966±1993) with the normalized means of the period 1961±1990.

10 212 A.W. Abdelhadi et al. / Agricultural Water Management 45 (2000) 203±214 Fig year moving average of mean monthly vapor pressure de cit for July, August and September (1966± 1993). use for CWR prediction. The increasing trend during these months may be explained by the severe drought conditions that prevailed in the region from the late 1970s. As these months represent the rainy season, the resulting low relative humidity due to the lack of rain combined with high temperatures led to increased evapotranspiration. This is in agreement with Mohamed (1998) who reported that the decline in rainfall in the Gezira area was attributed to the decline in July and August rainfall. This may further be clarified when the mean monthly vapor pressure deficit (10-year moving average for the period 1966±1993) of the concerned months are plotted as shown in Fig. 7. It is clear from Figs. 6 and 7 that care should be taken when the normalized means are used to predict CWR during the rainy season. However, knowing the fact that the rainy season coincides with the Blue Nile flood where shortages in irrigation water is not common, care must be taken only during prolonged dry periods. This is clear in case of August as the normalized ET 0 means were far below the actual trend due to the recent drought. This means that crops may need more water than what is estimated using the normalized means and K c values. 5. Conclusions The recommended Penman±Monteith reference crop evapotranspiration may be combined successfully with crop coefficients derived from crop phenomenological stages to predict CWR. Penman±Monteith method using ET 0 and K c was found to be better than Farbrother method that uses Penman E 0 and K f in the estimation of Acala cotton CWR under arid conditions in the Gezira scheme. The former method overestimated the total

11 A.W. Abdelhadi et al. / Agricultural Water Management 45 (2000) 203± CWR of Acala cotton by 3.4% and 2.5% when basal and derived K c were used, respectively, while the later overestimation was about 20%. The derivation of K c values would allow for the utilization of the recommended method in the arid and semiarid region of the whole central clay plain. This is very useful especially when the calculation of site-specific K c is expensive and time consuming for the large proposed irrigation projects along the Blue Nile. Examination of the recent weather trends showed that care must be taken when predicting the CWR in the Gezira region under drought conditions. Acknowledgements The author is indebted to the Japanese Government for the provision of the scholarship from the Ministry of Education, Science and Culture, which led to this work. Appreciation and gratitude are extended to the Director of the Agricultural Research Corporation, Gezira Meteorological Station staff and the scientists of the Gezira Research Station who provided the necessary information. References Adam, H.S., On the wind function in the Penman formula. In: F.O.A. Bailey (Ed.), Conference on Water Distribution in Sudanese Irrigated Agriculture, pp. 53±58. Adam, H.S., Farbrother, H.G., Crop-water use in irrigated and rainfed agriculture in the Democratic Republic of Sudan. United Nations Water Conference, Mar del plata, Argentina. E/CONF. 70/TP January 1977, pp. 1±25. Ahmed, S.A.H., Ahmed, K.E., Evapotranspiration in Sudan Gezira irrigation scheme. J. Irrigation Drainage Eng. 115 (6), 1018±1033. Ibrahim, A.A., Stigter, C.J., Ali, M.A., Hussein, S.A., Van Rheenen, W., On-farm sampling density and correction requirements for soil moisture determination in irrigated heavy clay soils in the Gezira, Central Sudan. Agric. Water Mgmt. 41, 93. Allen, R.G., Smith, M., Perrier, A., Pereira, L.S., An update for the de nition of reference evapotranspiration. ICID Bull. 43 (2), 1±92. Doorenbos, J., Pruitt, W.O., Guidelines for predicting crop water requirements. FAO irrigation and drainage paper no. 24. Food and Agriculture Organization of the United Nations, Rome, pp. 15±29, 112± 115. Doorenbos, J., Kassam, A.H., Yield response to water. FAO irrigation and drainage paper No. 33. Food and Agriculture Organisation of the United Nations, Rome, pp. 19±36. Elawad, O.M.A., Multicriterion approach to the evaluation of irrigation systems performance. Ph.D. Thesis, University of Newcastle upon Tyne, UK. Fadl, O.A., Evapotranspiration measured by a neutron probe on Sudan Gezira vertisols. Exp. Agric. 14, 341±347. Fadl, O.A., Water use of Acala cotton. Annual report 1978±1979. Gezira Research Station, Wad Medani, Sudan, pp. 143±147. Farbrother, H.G., Irrigation practices on Gezira clay-rates and intervals. Gezira miscellaneous paper no. 94. Gezira Research Station, Wad Medani, Sudan. Farbrother, H.G., Water requirements of crops in the Gezira. Annual report 1972±1973. Gezira Research Station, Wad Medani, Sudan, pp. 39±65. Farbrother, H.G., Modernization of indenting in the Gezira. In: Fadl, O.A., Charles, R.B., Conference on Water Distribution in Sudanese Irrigated Agriculture, pp. 78±93.

12 214 A.W. Abdelhadi et al. / Agricultural Water Management 45 (2000) 203±214 Ibrahim, A.M., Concepts of design and practice for irrigation distribution system in Sudan. In: Fadl, O.A., Charles, R.B., Conference on Water Distribution in Sudanese Irrigated Agriculture, pp. 107±116. Mohamed, H.A., Rainfall in the Sudan: trend and agricultural implications. Sudan Journal of Agricultural Research, 1, pp. 45±48. Penman, H.L., Natural evaporation from open water, bare soil and grass. Proc. R. Soc. London A 193, 120±146. Penman, H.L., Evaporation: an introductory survey. Neth. J. Agr. Sci. 4, 9±29.