Crop water requirement and availability in the Lower Chenab Canal System in Pakistan

Transcription

1 Water Resources Management III 535 Crop water requirement and availability in the Lower Chenab Canal System in Pakistan A. S. Shakir & M. M. Qureshi Department of Civil Engineering, University of Engineering and Technology, Lahore, Pakistan Abstract Lower Chenab Canal (LCC) is a perennial canal and conveys water to the fields through a network of branch canals, distributaries, minors and watercourses. The review of water supplies indicates a larger variation of discharges during the year and from year to year. The crop water requirement for the cultivable area of Lower Chenab Canal system is estimated using the state-of-the-art approaches. The approaches used for the estimation of reference evapotranspiration are reviewed. The review indicates that the Penman Monteith method is more suitable and provides reliable results compared to other methods. Other methods were also used but they seem to be less reliable in the light of previous investigations in this region. The mean monthly reference evapotranspiration is estimated using meteorological data of two stations (Faisalabad and Shorkot, the only reliable data collecting centres in the area), using the Penman-Monteith equation. The average annual reference evapotranspiration of the Lower Chenab Canal system for the period is estimated to be mm. The estimated values are maximum in June (203 mm) and minimum in December (43 mm) during this period. The area has an annual cycle of two main seasons named as Rabi (from mid October to mid April) and Kharif (from mid April to mid October). The average rainfall for the period is 200 mm with a maximum of 457 mm during year The estimated values of the consumptive use for Kharif crops are 300mm to 1450mm and for Rabi crops are 250 mm to 400mm. The actual canal irrigation water supplied (11 years average) for the LCC command area in Kharif season is about 5 BCM for a cultivated area of 0.9 million hectors and 4 BCM for Rabi crops for a cultivated area of 0.8 million hectors. These estimates are based on the data available with the Punjab Irrigation Department. The comparison of actual crop water requirement and water supplied indicates a shortage of more than 60% in Kharif and about 17% for Rabi crops. The study brings out that command area of the LCC System is under severe water shortage and is causing damaging effects on the irrigated agriculture. There is a need to enhance the water allocations for the area within the available water resources of the province. The better management and water conservation strategies can further decrease the gap between demand and supply. Keywords: reference evapotranspiration, Penman Monteith, crop coefficient, consumptive use, Lower Chenab Canal.

2 536 Water Resources Management III 1 Introduction The Lower Chenab canal off-takes from River Chenab from Khanki Head works. The command area of Lower Chenab Canal lies between latitude 32 o 09 and 32 o 15 N and longitude 73 o 02 and 73 o 55 E and approximately rectangular in shape (see figure 1). This is enclosed by Chenab and Ravi rivers on the northwest and southwest respectively and stretches up to about 80 Kilometres in length and 100 Km in width. It covers a gross area of 1.54 million hectors and cultivable command area of 1.25 million hectors, located in districts of Hafizabad, Seikhupura, Faisalabad, T.T. Singh, Jhang, Gujranwala, Gujrat, of the Punjab Province. The irrigation system is so constructed that there are no head gates at the outlets and if a particular canal has water in it, there is water in every watercourse on that canal. The farmers on the watercourse use the entire flow on the turn basis (warabandi). In other words, one has an allotted time interval flow based on the land holdings and has a fixed time each week. The farmers are allowed to divert whole watercourse flow onto their fields on their turn. The period of water supply is determined according to holdings of the farmers in the area to be irrigated by the outlet. The watercourse system does not always flow full, depending on supplies in the canals. Therefore, water available according to the warabandi system is not always constant. Figure 1: Location of command area of LCC Canal. 2 Cropping pattern for LCC system Cropping pattern is obtained by averaging eight years data ( ) of crop reporting available with Punjab Irrigation Department. The LCC bifurcates in two branch canals i.e. LCC East and LCC West and data for both of these canals

3 Water Resources Management III 537 is obtained separately. The cropping pattern for Rabi and Kharif crops of LCC system is shown in table 1. Table 1: Area of crops in percentage of total cropped area. 3 Data and information used in the study Mean monthly values for maximum and minimum temperature, wind speed, relative humidity and daily sunshine hours etc. from 1951 to 2002 were collected from the Pakistan Meteorological Department (PMD). The meteorological parameters are given in table 2&3 for two stations (Faisalabad and Shorkot) in the canal command area. The rational for selecting these two stations is that both stations approximately cover the whole area and also long time meteorological record is available only for these two stations. The crop duration, dates of planting and harvesting were determined on the basis of the agronomic character of different crops in Pakistan. The previous work done by Water and Power Development Authority (Revised Action Program, Lower Indus Project), Pakistan Agriculture Research Council (PARC), International Water Management Institute (IWMI), FAO 24, FAO 56, have been reviewed for the selection of crop coefficients. Data of water discharges supplied to the canal system are obtained from the records of Irrigation and Power Department from 1990 to Table 2: Meteorological data for the Station Shorkot.

4 538 Water Resources Management III Table 3: Meteorological data for the Station Faisalabad. 4 Estimation of crop water requirement Consumptive use of water or Evapotranspiration is one of the important elements of the hydrological cycle from the time water falls upon the land as precipitation until it reaches the ocean or it returns to the atmosphere. The subject of consumptive use, which includes evaporation of water from land and water surfaces and transpiration by vegetation, is becoming increasingly significant, particularly in arid and semi arid irrigated areas of the world [7]. The different components of consumptive use at canal command level in Lower Chenab Canal System were estimated. Estimation of reference Evapotranspiration was made based on several years available climatic data (from 1951 to 2002). Computations of reference Evapotranspiration were made at canal command level and compared with other methods including the Hargreaves method. Cropping periods were estimated based on literature review and information obtained from the interviews of formers and staff of the agriculture department in the study area. Crop coefficients were calculated using the procedure given in FAO 56 [2]. The estimation of consumptive use was made considering the actual cultivated area of each crop and crop coefficients developed earlier. The availability of actual water to the crops was calculated from the water supplied by the Punjab irrigation Department during the period and comparisons are made with compatible units. 4.1 Reference evapotranspiration Evaporation and transpiration occur simultaneously and there is no easy way of distinguishing between the two processes. Evaporation is the process whereby liquid water is converted to water vapour (vaporization) and removed from the evaporating surface (vapour removal). Transpiration consists of the vaporization of liquid water contained in plant tissues and the vapour removal to the

5 Water Resources Management III 539 atmosphere. According to the FAO 56, following definition of the reference surface is accepted, The reference surface is a hypothetical grass reference crop with an assumed crop height of 0.12 m, a fixed surface resistance of 70 sm -1 and an albedo of 0.23 [2]. The reference surface closely resembles an extensive surface of green, well- watered grass of uniform height, actively growing and completely shading the ground. For estimation Eto by Penman-Monteith method the basic equation is given below: ETo = [0.408 (Rn - G) + γ(900u 2 ) (e s - e a )/(T mean +273)] / [ + γ (1+0.34U 2 )] Where Eto reference evapotranspiration (mm/day) Rn net radiation at the crop surface (MJ/m 2 day) vapour pressure curve (Kpa/ o C) G soil heat flux density (MJ/m 2 day) γ psychrometric constant (Kpa/ o C) U 2 wind speed at 2 m height (m/s) e s saturation vapour pressure (Kpa) e a actual vapour pressure (Kpa) e s - e a saturation vapour pressure deficit T mean mean daily temperature ( o C) The mean monthly reference evapotranspiration is estimated using meteorological data of two stations Faisalabad & Shorkot as discussed above. The comparison of values thus calculated using Penman-Monteith method and Hargreaves methods is presented in table 4. It indicates that Hargreaves method over estimates the reference evaporation values many times compared with Penman Monteith method. The review of literature suggests that Penman Monteith method gives relatively better estimates compared with Hargreaves or any other method for this region [5]. Table 4: Comparison of monthly reference evapotranspiration (mm). 4.2 Cropping coefficients Kc Crop coefficient is basically the ratio of the actual crop evapotranspiration and the reference evapotranspiration. It represents an integration of the effects of four primary characteristics that distinguish the crop from the reference crop. These

6 540 Water Resources Management III include crop height, albedo, canopy resistance and evaporation from the soil. The value of Kc for a given crop varies with the crop growth stages. As described in the FAO 56, four crop-growth stages are considered, (i.e. initial stage, crop development stage, midseason stage and late season stage). The calculation of crop coefficients for LCC System involved the selection of crop period and development of Kc curves for different crops. The cropping period varies for individual crops but is generally associated with Rabi and Kharif. Rabi crops are sown after the rainy season in mid of October to November, and are harvested in spring in April and May. Wheat, oilseed and fodder are the main crops of the Rabi season, and cotton, rice, maize, vegetables and fodder are main crops of Kharif season in this area. Sugar cane is an annual crop, and has two different crop calendars within the area. Figure 2: Kc curve for wheat for LCC. Figure 3: Kc curve for cotton for LCC. The periods of planting and harvesting, crop duration and crop growth stages for the LCC System have been estimated by the direct or indirect information

7 Water Resources Management III 541 about cropping practices in the area. The Kc curves developed for wheat and cotton (the main crops) are compared with IWMI curves developed for areas in the vicinity of this canal system and show a reasonable comparison. These are shown in figure 2 and 3 respectively. 4.3 Consumptive use of water for LCC system The consumptive use can be defined as quantity of water, in hector-meter per cropped hector per year, absorbed by a crop and transpired or used directly in the building of plant tissues, together with that evaporated from the crop-production land [2]. It can be estimated as the product of reference evapotranspiration, crop coefficient and crop calendar. The estimated values are given in table 5. For the existing cropping pattern the consumptive use of water has been calculated by multiplying crop Evapotranspiration in the LCC System with their respective cropped area. The water requirement for the Rabi and Kharif crops is shown in table 6. Table 5: Consumptive use for LCC system. Table 6: Crop water requirement/consumptive use for LCC system.

8 542 Water Resources Management III Table 7: Comparison of consumptive use with actual water supplied for LCC system. Figure 4: Comparison of actual and required water supply for Kharif season. Figure 5: Comparison of actual and required water supply for Rabi season.

9 5 Calculation of actual supply to crops Actual water supply in hector meter per hector for the Kharif and Rabi season is calculated based upon the data obtained from the Punjab Irrigation Department and is compared with the water requirement of the crops estimated above. Results are given in table 7 and are shown graphically in figure 4 and 5. 6 Discussions The above analysis indicates that LCC Command area is under serious water shortage especially in the Kharif season. However, it is reported that farmers make use of ground water to over come these shortages, which was not included in the analyses. It may be further noted that ground water is not regulated by the government agencies and farmers extract these supplies with the help of shallow tube wells at their own cost. The quality of ground water in this area is generally not good and is causing added salinity problems due to these interventions. It may be emphasized that overcoming the deficiency of water by the farmers using low quality ground water does not necessarily increase the production to such a level as may be possible with good quality canal water due to increased salinity effects in the soils. As discussed above, the command area of LCC System is under severe water shortage and is causing damaging effects on the irrigated agriculture. There is a need to enhance the water allocations for the area within the available water resources in addition to make efforts to provide the committed water supplies to the farmers. The better management and water conservation strategies (e.g. lining of canals and water courses, improved agricultural practices and encouraging low delta crops) can further decrease the gap between demands and supply and can have a positive impact on the agro-economics of the area. 7 Conclusions and recommendations Water Resources Management III 543 The following conclusions can be drawn from this study, The Penman-Monteith method as described in FAO 56 seems to be more suitable for estimation of the reference Evapotranspiration for the areas like LCC Command (where humidity is more than 60%). LCC Command area is under severe water shortage of about 17% in Rabi and more than 60% during the Kharif season. The shortage is partly met with low quality ground water, which is causing damaging effects on the irrigated agriculture of the area, particularly the increase in salinity and lower yields of the crops. It is recommended that water allowance for the LCC command area should be increased to minimise the ill effects of the shortage of good quality canal water. The better water management and conservation strategies are needed to discourage the extraction of low quality ground water in the area.

10 544 Water Resources Management III References [1] FAO, Crop Water Requirement, Irrigation and Drainage Paper No.24, Rome [2] FAO, Crop Evapotranspiration, Guidelines for Computing Crop Water Requirement, FAO Paper No. 56, Rome. [3] George H. Hargreaves, Accuracy of Estimating Reference Crop Evapotranspiration, Journal of Irrigation and Drainage and Engineering, ASACE, Vol. 115, No. 6, December, [4] George H. Hargreaves, Accuracy of Estimating Reference Crop Evapotranspiration, Journal of Irrigation and Drainage and Engineering, ASACE, Vol. 120, No. 6, December, 1994 [5] IWMI, 2001, Spatial Distribution of References and Potential Evapotranspiration, Working Paper 24 Pakistan Country Series Number 8. [6] LIP, Principals and Criteria for Future Development: Volume 18, WAPDA, Hunting Tech. Services Ltd., Sir M. MacDonald and Partners, Pakistan. [7] Marvin E. Jensen, Consumptive Use of Water and Irrigation Water Requirements 1971, American Society of Civil Engineers New York. [8] PARC, Agro-Ecological Regions of Pakistan, Pakistan Agriculture Research Council, Islamabad. [9] PARC, Consumptive Use of Water for Crops in Pakistan, Pakistan Agriculture Research Council, Islamabad. [10] RAP, 1979 Revised Action Programme for irrigation Agriculture, WAPDA, Pakistan.