HYDRODYNAMIC MODELLING FOR WATER-SAVING STRATEGIES IN IRRIGATION CANALS y

IRRIGATION AND DRAINAGE Irrig. and Drain. (2008) Published online in Wiley InterScience (www.interscience.wiley.com).375 HYDRODYNAMIC MODELLING FOR WATER-SAVING STRATEGIES IN IRRIGATION CANALS y MUHAMMAD ZUBAIR KHAN 1 AND ABDUL RAZZAQ GHUMMAN 2 * 1 Department of Water Management, NWFP Agricultural University Peshawar, Pakistan 2 University of Engineering and Technology, Taxila, Pakistan ABSTRACT In this paper a hydrodynamic model was applied for evaluation of water-saving strategies in systems where there is wastage of water due to oversupply in periods of low demand. The one-dimensional hydrodynamic model CanalMan was used to assess water saving in a secondary canal. Different operational scenarios were evaluated in comparison with the existing operational strategy. This included the operation of the canal at different discharges combined with night closure, in periods of low demand. Time discharge hydrographs were prepared using the numerical model for operation of canals having daily opening and closing with full supply, 80 and 70% of design discharge. These hydrographs were used to estimate possible time duration of canal operation for these discharges in the case of night closure. Water saving by night closure was compared with that from operation having weekly rotations. Study of the existing operational practices indicated extremely high oversupply to the canal compared with crop water requirements, especially in the months of August, November and December. Significant water saving was found by night closure and operating the canal at reduced discharges. Copyright # 2008 John Wiley & Sons, Ltd. key words: CanalMan; night closure; hydrographs; irrigation; Pakistan; NWFP; canals; water saving; mathematical modelling Received 6 April 2007; Revised 11 October 2007; Accepted 26 October 2007 RÉSUMÉ Dans ce papier un modèle hydrodynamique a été appliqué pour évaluer les stratégies d économie d eau dans les systèmes où on constate un gaspillage d eau dû à une alimentation excessive en période de faible demande. Le modèle hydrodynamique 1D CanalMan a été employé pour évaluer l économie d eau dans le canal secondaire. Différents scénarios opérationnels ont été évalués par rapport à la stratégie opérationnelle existante, incluant l exploitation du canal à différents débits et la fermeture nocturne en période de faible demande. Des hydrogrammes ont été élaborés en utilisant le modèle numérique pour le fonctionnement des canaux avec ouverture et fermeture quotidiennes et alimentation à plein débit, 80 et 70% du débit. Ces hydrogrammes ont été employés pour estimer la durée possible d exploitation du canal pour ces débits en cas de fermeture nocturne. L économie d eau par fermeture nocturne a été comparée avec celle qui résulte d une exploitation en rotation hebdomadaire. L étude des pratiques opérationnelles existantes a montré l extrême suralimentation du canal comparée aux besoins des plantes, particulièrement en août, novembre et décembre. Une économie d eau significative est possible grâce à la fermeture nocturne et à l exploitation à débit réduit. Copyright # 2008 John Wiley & Sons, Ltd. mots clés: CanalMan; fermeture nocturne; hydrogrammes; irrigation; Pakistan; NWFP; canaux; économie d eau; modèle mathématique * Correspondence to: Abdul Razzaq Ghumman, Professor, Civil Engineering, University of Engineering and Technology, Taxila, Pakistan. E-mail: abdulrazzaq@uettaxila.edu.pk y Modèle hydrodynamique pour des stratégies d économie de l eau dans les canaux d irrigation. Copyright # 2008 John Wiley & Sons, Ltd.

M. Z. KHAN AND A. R. GHUMMAN INTRODUCTION There are many areas where irrigation systems are unable to provide the required water supply at tail ends. In contrast to this, irrigation canals designed to meet peak irrigation requirements usually have excess water supply in periods of low demand, which results in wastage of water. There is a need to investigate strategies for optimum use of irrigation water so that excess water may be supplied to areas of water scarcity. Efficient operation and management of the system are the feasible solution for this (Mishra et al., 2001; Ghumman et al., 2006). In traditional large-scale supply-based irrigation systems the objective is to irrigate the maximum area with the limited water supply with design cropping intensities of 75%. Recently in Pakistan in the field of irrigation system development, efforts have been made to change some of the supply- based systems into crop-based systems. Canals in these systems are designed to carry discharges which can meet water requirements even in the peak summer season for cropping intensities of up to 180%. The operation of such systems is the responsibility of the Irrigation Department. They try to interfere as little as possible with the system. As a result there is oversupply especially in the head reaches of the system and a lot of wastage of water in periods of low demand. In such a situation an important response of the water users (farmers) is the abandonment of night irrigation. Chambers (1988) and Bievre et al. (2003) discovered that most of the canal water continues to flow at night, and much of it is poorly used or wasted. Darkness, cold, fear, and desire for sleep deter irrigation staff and farmers from activities at night. Chambers (1988) suggested four measures to reduce irrigation at night with water saving. These all involve storage of water that can be achieved in different ways; storage in main reservoirs, canals, intermediate storage reservoirs, on-farm reservoirs, and in groundwater. A lot of research work is being done on irrigation systems in different ways. Smout and Gorantiwar (2006) have worked on productivity and equity of different irrigation schedules under limited water supply. Lecina and Playán (2006) have developed and applied a model for the simulation of water flows in irrigation districts. De Vries and Anwar (2006) have worked on irrigation scheduling giving importance to travel times. Merriam and Freeman (2007) have studied operational cost benefits of flexible on-farm irrigation supply systems. As per the authors literature survey, no in-depth work has so far been done on water-saving strategies by night closure of canals. Bievre et al. (2003) have done work on night irrigation reduction for water saving in medium-sized systems. Water saving by total closure of canals at night was not investigated. Ghumman et al. (2006) and Khan (2006) applied the CanalMan model, developed by the Utah State University, USA, to a secondary canal in the Abazai branch of the Upper Swat Canal (USC) Irrigation System in the North West Frontier Province (NWFP) of Pakistan to assess its hydraulic behavior. Modifications to outlets were investigated with regard to better performance of the canal. The idea of night closure was discussed but was not investigated in depth. The present study deals with the assessment of the operational practices of the Irrigation Department and investigates possible water-saving strategies. It compares different options of operating canals at various discharges to save water so that the water supply into the secondary canal is both adequate and equitable. Water saving by night closure was investigated in depth in combination with the different discharge options. The site already investigated by Ghumman et al. (2006) was selected to study water saving by night closure with and without incorporating the improvements suggested by Ghumman et al. (2006). For a ready reference a brief description of study site is given below. SHINGRAI MINOR AND RELATED DATA The Shingrai Minor is located on the Abazai branch of the USC (Upper Swat Canal), in NWFP, Pakistan. Detailed overview of the USC can be seen from Figures 1 and 2 as given in Ghumman et al. (2006) and Khan (2006). Shingrai Minor is an offtake on the left side of the Abazai branch of the USC at reduced distance (RD) 12 599 m and is the first major secondary canal. It is representative of the rest of the secondary canals of the system in design. It represents medium-sized secondary canals of the USC. It has a length of 5289 m and design discharge of 0.84 m 3 s 1. It has 13 outlets of which 11 are short-crested crump weirs, and 2 open flumes. All the outlets are fixed structures with no adjustable parts, and discharge through these outlets is entirely dependent on the water depth above their sills. It has a culturable command area of 1170 ha. Most of the canal is lined, with a trapezoidal shape and a longitudinal slope of 0.0005.

HYDRODYNAMIC MODELLING FOR WATER-SAVING STRATEGIES Figure 1. Map of the Upper Swat Canal (Ghumman et al., 2006) Necessary data required were collected partly through field measurements and partly from the Irrigation Department, NWFP Pakistan (Shah, 2000; Khan, 2006). Water depths in the canal along all the outlets and the canal head regulator were collected. The head regulator and all the outlets were calibrated through regular measurement of discharges and their coefficients of discharge determined. All the canal design dimensions data were collected from the Irrigation Department and verified by measurements in the field. The period of data collection was August December 1999. The geometric and hydraulic data were reconfirmed in December 2004 through additional survey and field measurements (Khan, 2006). Irrigation requirements were taken from the Swabi Salinity Control and Reclamation Project (Swabi SCARP, 1991). The gross irrigation requirements at secondary level were determined using an efficiency of 75%.

M. Z. KHAN AND A. R. GHUMMAN Figure 2. Schematic map of Shingrai Minor, (Ghumman et al., 2006) EXISTING OPERATION OF SHINGRAI MINOR A comparison of the gross irrigation requirements and the supply indicates oversupply to the Shingrai Minor from August to December (Figure 3). In August due to monsoon rains the demand remains low. In the period October to December, despite low demand, at least one irrigation is required by most of the farmers to practice their first irrigation of wheat crop after sowing or fill the root zone of fodder crops, before the annual closure of the system in January. Shingrai Minor was mostly operated at full supply during the five months of monitoring. Interference by the Irrigation Department was at a minimum as far as operation of the canal was concerned, and small variations in discharges were a result of fluctuations in the supply in the main canal. There were rotations of one-week duration (one week closed and one week operative) in November and two weeks rotation in December. CANALMAN MODEL CanalMan software solves the Saint-Venant equations of continuity and motion (Strelkoff, 1969) for one-dimensional unsteady open-channel flow in a branching canal system using finite differences scheme (Merkley, 1997). The model is highly robust. The model can be used to simulate various canal operations. There are many common local gate automation schemes in the model. One can start simulations by filling an empty canal Figure 3. Supply and demand of Shingrai Minor

HYDRODYNAMIC MODELLING FOR WATER-SAVING STRATEGIES system, continuing from a previous simulation, or from a specified steady or unsteady flow condition. Boundary conditions are specified at locations of flow control structures, or other places where the flow depth and flow rate are significantly affected by the presence of a structure (Murkley, 1997). The model, like all other models, also has some limitations. As described in its manual, it cannot analyze rapid flow changes, channel de-watering, negative flow at in-line structures, hydraulic jumps, surges, bores, supercritical flow and looping canal systems. The computational time step can be from 1 to 10 min, in whole minutes. Channel cross-sections can be circular or trapezoidal. The model cannot be applied to a nonprismatic channel within a reach; however, the cross-sectional shape and size can change from reach to reach, but not within a reach. To fix the computational length a certain number of nodes are used internally by the model. This model was selected for the study for the following reasons: it has the ability to start a simulation from an initial dry bed condition, because major operational strategies studied were frequent opening and closing of canals; its calibration and validation was easily done as given below. MODEL CALIBRATION AND VALIDATION The calibration and validation made by Ghumman et al. (2006) and Khan (2006) were used in this study. The model calibration was done using simulated and measured water levels in the canal reaches. Canal discharge was specified at the entrance as the upstream boundary condition. The canal was assumed to attain a steady state after the discharge value reached 846 l s 1. The computational time step was taken as 1 min. A total of 20 nodes were specified for every reach. Rating curve equations were used as boundary conditions at the outlets. Optimal values of the Manning s roughness coefficient (n) and outlet discharge coefficients (C d ) were determined so that simulated values become close to the measured values. The head regulator and outlets were calibrated by development of head discharge relationships between water levels in the canal above the crest of the outlets and discharges through the outlets. Another data set collected at 750 l s 1 was used for validation of the model for Shingrai Minor. Model accuracy for the calibration and validation were determined by calculating the model efficiency (ME) as follows: ME ¼ðST SEÞ=ST where ST ¼ P (simulated depth ( P measured depth)/n) 2 SE ¼ P (simulated depth measured depth) 2 The results of model calibration were as follows: the Manning n values ¼ between 0.016 0.018 for lined reaches and 0.023 for unlined reaches; model efficiency ¼ 92%; the difference between the simulated and measured outlet discharges ¼ less than 5 l s 1. For validation of the model the model efficiency was 95%. Graphs for calibration and validation results can be seen in Ghumman et al. (2006) and Khan (2006). TEST RUNS FOR WATER SAVING Normally in periods of low irrigation demand alternate weekly rotational supply is introduced to the secondary canals of the irrigation system by the Irrigation Department. Operational strategies by night closure and reduced discharges were assessed and compared to weekly rotational supply. The following five different operational modes

M. Z. KHAN AND A. R. GHUMMAN Table I. Actual and modified outlet dimensions (after Ghumman et al., 2006) Outlet no. Outlet type Actual width (cm) Modified width (cm) Outlet no. Outlet type Actual width (cm) Modified width (cm) 1R a Crump weir 21.0 17.0 7R Crump weir 21.0 16.0 2R Crump weir 30.0 26.0 8R Crump weir 25.5 27.0 3L b Crump weir 23.5 16.0 9L Crump weir 18.0 18.0 4L Open flume 8.7 8.5 10L Crump weir 15.0 12.5 5R Open flume 8.2 11.0 11R Crump weir 45.0 30.0 6R Crump weir 62.0 62.0 12TL Crump weir 18.0 14.0 a R ¼ right outlet. b L ¼ left outlet. were assessed from the point of view of water saving. Improvements in outlet dimensions suggested by Ghumman et al. (2006) shown in Table I were considered in these modes: night closure at design full supply without improvements in outlet dimensions; night closure at design full supply with improvements in outlet dimensions; night closure at 80% of full supply with improvements in outlet dimensions; night closure in combination with 70% of the design discharge with improvements in outlet dimensions; weekly rotational supply to the canal outlets at the above-mentioned (100, 80, 70%) discharge levels. SIMULATED HYDROGRAPHS FOR NIGHT CLOSURE The outlet hydrographs under operation of the canal at 100, 80 and 70% of the design discharges are presented in Figures 4, 5, 6 and 7. Figure 4 represents simulation of the canal at full supply without improvements in the dimensions of outlets suggested by Ghumman et al. (2006), given in Table I. The operation time required is 9 h. Figures 5 7 represent simulation of the canal with the above-mentioned improvements. The time required to attain steady state by the canal outlets increases with the decrease in discharge level. It is 210, 240 and 270 min at the 100, 80 and 70% discharges respectively. After the canal supply is cut off, water is still available to the tail outlets for 270, 210 and 180 min at the three discharge levels respectively. The water-saving simulations were made Figure 4. Hydrograph at 100% (0.846 cumecs) without improvements in outlet dimensions

HYDRODYNAMIC MODELLING FOR WATER-SAVING STRATEGIES Figure 5. Hydrograph at 100% (0.846 cumecs) with improvements in outlet dimensions for durations of 9 h (without changes in outlet sizes) for full supply and 6, 10 and 12 h (with changes) for three discharges respectively, which is the time required to meet the irrigation demand of the outlets at these discharges. WATER SAVING BY DIFFERENT OPERATIONAL OPTIONS As discussed earlier, some kind of flow reduction measures is required when the potential supply exceeds the irrigation demand. In the case of the Shingrai Minor the months of November and December can be identified as the period where water saving can be made by reducing supply and by night closures. Figure 6. Hydrograph at 80% (0.677 cumecs) with improvements in outlet dimensions

M. Z. KHAN AND A. R. GHUMMAN Figure 7. Hydrograph at 70% (0.590 cumecs) with improvements in outlet dimensions Night closure at full supply discharge without improvements in outlet dimensions For night closure at full supply discharge without improvements in outlet dimensions (existing real situation) an operation of 9 h daily was required to adequately supply water to all the outlets Figure 8 compares night closure and weekly rotational supply at 100% discharge (0.846 m 3 s 1 ). Some oversupply even in the case of night closure is due to the under-performance of some of the outlets, and in order to satisfy the demand of those outlets other outlets had to be oversupplied. The underperformance of outlets at different discharges has been discussed by Ghumman et al. (2006). Even without improvements in the outlet dimensions the option of night closure of the canal appeared to be feasible, and significant saving on water was observed as compared to weekly rotation as suggested by the Irrigation Department of NWFP, Pakistan (Figure 8). The average oversupply was estimated as 52 and 164% for night closure and weekly rotational operation respectively. Night closure at full supply discharge with improvements in outlet dimensions The comparison of water saving at the design full supply and alternate weekly rotational supply is presented in Figure 9. It indicates that the canal outlets will be highly oversupplied even in the case of rotational supply with an Figure 8. Comparison of Night closure (9 hours daily operation) and weekly rotational supply at 100% supply without improvements in outlet dimensions (existing real situation)

HYDRODYNAMIC MODELLING FOR WATER-SAVING STRATEGIES Figure 9. Comparison of Night closure (6 hours daily operation) and weekly rotational supply at 100% (0.846 cumecs) with improvements in outlet dimensions average excess of 104%. Alternatively, operation of the canal for only 6 h daily in the case of night closure will be adequate and the average over- supply will be only 4%. Water saving in this case will thus be up to 100%. This option may be much liked by the farmers and at the same time saves water significantly. Night closure at 80% of full supply discharge with improvements in outlet dimensions Figure 6 shows that a daytime operation of 9 h is required to meet the daily irrigation demand of the outlets in the case of night closure. The comparison of weekly rotational supply and night closure of the canal at this discharge level (Figure 10) shows that the oversupply will be 60 and 20% respectively. Thus there can be water saving of 40% by opting to close the canal at night as compared to weekly rotations, which in addition include difficulties associated with night irrigation. Night closure at 70% of full supply discharge with improved outlets The comparison at 70% discharge level is presented in Figure 11. An operation of 12 h would be required daily to meet the irrigation demand of the outlets. This option is essentially the same as weekly rotational supply at this discharge level. The average oversupply was observed to be 29%. Figure 10. Comparison of Night closure (9 hours daily operation) and weekly rotational supply at 80% (0.677 cumecs) with improvements in outlet dimensions

M. Z. KHAN AND A. R. GHUMMAN Figure 11. Comparison of Night closure (12 hours daily operation) and weekly rotational supply at 70% (0.590 cumecs) with improvements in outlet dimensions The above discussion clearly indicates that canals similar to Shingrai Minor (medium and small-sized canals) can be easily closed at night in order to save water at 100 and 80% discharge levels. Option of night closure at full supply is preferred because it requires only 6 h of operation. In this case all the activities of farmers and irrigation staff will be finished in daytime. Since there is a tendency of users not to use irrigation at night in the winter months, canal closure at night may be considered a feasible option. SUMMARY AND CONCLUSIONS In irrigation systems designed to meet peak crop water requirements, significant water saving can be made in periods of low demand. The operational strategies of the Irrigation Department have been compared with alternate operations of a demand-based canal by numerical modelling. It illustrates the usefulness of hydrodynamic modelling for evaluation of water-saving strategies. Usefulness of the one-dimensional hydrodynamic model CanalMan has been demonstrated by its application to a secondary canal in the Upper Swat Canal Irrigation System in the North West Frontier Province of Pakistan. Water saving through night closure of the canal was assessed and compared to the weekly rotational supply practiced by the Irrigation Department. Different night closure scenarios evaluated included simulation of the operation of the canal at 100, 80, and 70% of the full supply. It was concluded that the scenario of operating the canal for 6 h at daytime and closing the canal at night was the best option. The options of operating the canal at lower discharges will also require some hours of night irrigation and hence were not recommended, although even these options can save water. The following main conclusions were drawn from the study: Shingrai Minor is operated as supply based. For most of the period the supply to the canal was higher as compared to the demand; numerical modelling was found to be suitable for evaluation of the operation of a secondary canal for different discharges and night closure; water-saving operational strategy of night closure was found to be feasible for small and medium-sized canals; significant water saving can be made by night closure in periods of low demand for the canals designed to meet the peak irrigation requirements.

HYDRODYNAMIC MODELLING FOR WATER-SAVING STRATEGIES RECOMMENDATION When canals have to be operated at lower discharges according to the irrigation demand, higher silt deposition will be likely. This has not been studied in the present study. Sediment transport study is therefore recommended for the future to investigate the potential problem. ACKNOWLEDGEMENTS We would like to thank the Biological and Irrigation Department, Utah State University, Logan, Utah, USA, for making the CanalMan software freely available, which was used in this study. The cooperation of the Irrigation Department was also greatly valued in providing the design data and drawings of the Shingrai Minor. Support of the Higher Education Commission Islamabad, Pakistan, is gratefully acknowledged. REFERENCES Bievre B, Alvarado A, Timbe L, Celleri R, Feyen J. 2003. Night irrigation reduction for water saving in medium-sized system. Journal of Irrigation and Drainage Engineering, ASCE 129(2). Chambers R. 1988. Managing Canal Irrigation: Practical Analysis from South Asia, Institute of Development Studies, Oxford, and IBH Publishing Co. Ltd. De Vries TT, Anwar AA. 2006. Irrigation scheduling with travel times, Journal of Irrigation and Drainage Engineering, ASCE, 132(2): 220 227. Ghumman AR, Khan MZ, Khan MJ. 2006. Use of numerical modelling for management of canal irrigation water. Irrigation and Drainage 55: 445 458. Khan MZ. 2006. Investigation of optimal operation strategies for irrigation systems. PhD dissertation, Department of Civil Engineering, University of Engineering and Technology, Taxila, Pakistan. Lecina S, Playán E. 2006. Model for the simulation of water flows in irrigation districts. I: Description. Journal of Irrigation and Drainage Engineering, ASCE 132(4). Merkley GP. 1997. CanalMan: A Hydraulic Simulation Model for Unsteady Flow in Branching Canal Networks, User s Guide. Dept. of Biological and Irrigation Engineering, Utah State University, USA. Merriam JL, Freeman J. 2007. Operational cost benefits study of flexible on-farm irrigation supply systems. Journal of Irrigation and Drainage Engineering, ASCE 132(3). Mishra A, Anand A, Singh R, Raghuwanshi NS. 2001. Hydraulic modelling of Kangsabati Main Canal for performance assessment. Journal of Irrigation and Drainage Engineering, ASCE 127(1). Shah M. 2000. Hydraulic performance evaluation of Shingrai Minor of Upper Swat Canal Irrigation System. MSc thesis, Department of Water Management, NWFP Agricultural University, Peshawar, Pakistan. Smout IK, Gorantiwar SD. 2006. Productivity and equity of different irrigation schedules under limited water supply. Journal of Irrigation and Drainage Engineering, ASCE 132(4): 349 358. Strelkoff T. 1969. One-dimensional equations of open-channel flow. Journal of the Hydraulics Division, ASCE 95(HY3): 861 876. Swabi SCARP. 1991. Irrigation water requirements. Working Paper No. 3. Swabi Salinity Control and Reclamation Project Consultants, Peshawar, Pakistan.