Chapter 3 – Hydrology and sediment transport 27 Chapter 3 Hydrology and sediment transport SUMMARY In developing a spate system it is important to understand the entire hydrology of the system – the base flow, sub-surface flow and groundwater and the pattern of spate floods that will dictate the potential yield of spate systems, the design of diversion structures and canals and the area to be potentially irrigated. Spate hydrology is characterized by a great variation in the size and frequency of floods, which directly influence the availability of water for agriculture in any one season. Spate floods can have very high peak discharges and are usually generated in wadi catchments by localized storm rainfall. Crop production varies considerably because of the large variation in wadi runoff from year to year, season to season and day to day. The extreme characteristics of wadi hydrology make it very difficult to determine the volumes of water that will be diverted to fields and hence the potential cropped areas. Wadis transport very high sediment loads which can be two or more orders of magnitude larger than those encountered in most runoff river perennial irrigation systems. Management of sedimentation is, therefore, a key factor in spate irrigation and must be given particular consideration in designing spate projects. Hydrological and sediment transport data are needed to design improved water diversion structures and canals in spate schemes and to estimate the cropped area that can be potentially reached by spate. These data include the annual volumes of water available at the diversion point(s); the probable distribution of spate runoff events; the distribution of flows during runoff events; the proportion of the annual hydrograph that occurs in different flow ranges; wadi bed seepage rates; the magnitude and return periods of extreme discharges for the design and protection of the permanent works; the concentrations and size range of the sediments transported by spate events and their relationship with wadi discharges; and the sediment-transporting capacity of existing canals. In particular, the distribution of discharges within the annual runoff has a large impact on the water diversion strategy that will be adopted, particularly with regard to the relative importance of seasonal base flows. In most schemes, the long-term data that would be needed to provide the information listed above is unavailable. Unless a period of hydrological and sediment data collection, combined with numerical flow and sediment transport models is possible, the estimation of the above variables must be made through the use of empirical methods combined with good hydrological judgement. Table 3.1 lists some of the methods used to collect and analyse the hydrological and sediment transport information required to design improved intakes and canal networks. They are described in the following sections. 28 Guidelines on spate irrigation The calculation of mean annual runoff through a simple runoff coefficient, combined with the use of non-dimensional flow duration curves, makes it possible to estimate the volumes of water that can be diverted and design spate intakes accordingly. Such curves and coefficient depend on the characteristics of the catchments and local climate and care must be taken in applying them to ungauged catchments. Local knowledge can greatly contribute to the assessment of hydrological characteristics of wadi catchments and is often the only source of information. Farmers in the wadi can provide information on the number and sizes of floods and their variations between years, thus making it possible for the hydrologist to establish flood-frequency curves. More important is the use of local knowledge for the establishment of potentially cropped areas. In areas where traditional spate irrigation exists, farmers can determine the area to be irrigated on the basis of their past experience and from observation of the quantities of water diverted by any improved diversion and conveyance arrangements. This involves surveys to determine the extent of the existing irrigated areas. Surveys have to be combined with local knowledge and supplemented by interviews with farmers to establish how often fields in different parts of the system are irrigated and how this varies from year to year. When new areas are being developed, irrigation engineers and agronomists need to determine the potential area that can be irrigated and the capacities of the canals that will be needed through estimates of the proportion of annual runoff that will be diverted, its distribution in time, and the characteristics of the area to be cropped (including soil water-holding capacity). Crop water requirements, while they provide a useful estimate of the maximum volumes of water required, will usually not be the main factor in assessing the potential irrigated area, as farmers will seek to expand their land under irrigation to the maximum possible extent. Another important characteristic of wadi hydrology is the high rate of infiltration of floodwater in the wadi bed, with many small floods not reaching the lower reaches of the wadi. Seepage in the wadi bed is often the only source of groundwater recharge. Consequently, what is often considered ‘loss’ for spate through seepage may very well be used in a very productive way through groundwater extraction. Similarly, when spate intakes divert a substantial part of the wadi flow, they impact groundwater recharge downstream with possible negative implications for communities relying on groundwater. A river basin approach to spate irrigation planning is therefore necessary, to ensure that any intervention results in an overall increase in benefits for the populations of the wadi, and avoids losses for water users downstream (see Chapter 10). Chapter 3 – Hydrology and sediment transport 29 INTRODUCTION Spate hydrology is characterized by a great variation in the size and frequency of floods which directly influence the availability of water for agriculture. Wadis are also characterized by very high sediment loads and important groundwater recharge through seepage in the wadi bed. All these characteristics are specific to wadi hydrology. Management of floods and high sediment load therefore require a good estimate of the main hydrological characteristics of the wadi. This chapter presents a brief description of runoff and sediment transport processes that influence spate irrigation practices and the design of improved spate irrigation schemes. It also provides some simple methods that can be used to derive the hydrological information needed to design intakes and canals for spate irrigation systems. The emphasis is on methods used for small schemes, where little data are available and the specialist hydrological studies that are carried out in support of larger projects are not feasible. The results derived with these methods should be verified wherever possible by comparison with any local or regional data that may be available. DATA REQUIREMENTS Hydrological and sediment transport data are needed to design improved water diversion structures and canals in spate schemes. The following information should ideally be available to designers of intakes and canals: Z the annual volumes of water available at the diversion point(s) in terms of seasonal incidence and reliability; Z the probable distribution of spate runoff events in terms of peak flows and flood volumes; Z the distribution of flows during runoff events, particularly the shape of the recession limb of the hydrograph, which provides the bulk of the water that can be diverted to irrigation command areas; Z the proportion of the annual hydrograph that occurs in different flow ranges (flow duration curve); Z wadi bed seepage rates; Z the magnitude and return periods of extreme discharges for the design and protection of the permanent works; Z the concentrations and size range of the sediments transported by spate events and their relationship with wadi discharges; and Z the sediment-transporting capacity of existing canals. In most schemes, the long-term data needed to provide the information listed above are unavailable. Major spate irrigation improvement projects thus include a short period of hydrological and sediment data collection. The data are often used to assist in validating numerical flow and sediment transport models. For small- and medium-scale schemes data requirements are smaller, and simpler methods requiring minimal field data are appropriate. Maximum use needs to be made of the local knowledge that farmers have. Table 3.1 lists some of the methods used to collect and analyse the hydrological and sediment transport information required to design improved intakes and canal networks. They are described in the following sections. WADI HYDROLOGY AND IMPLICATION FOR SPATE DESIGN The high-intensity rainfall events that generate spate flows in wadis are characterized by a wide variability in space and time. Information on the spatial characteristics of 30 Guidelines on spate irrigation TABLE 3.1 Hydrological and sediment transport information collection methods Parameter Method Remarks Seasonal/annual Long-term discharge data from UÊ Rarely if ever available. discharge and flow-gauging station Needs properly sited and maintained gauging station. probabilities of UÊ occurrence UÊ Discharge usually computed from continuous water level records and derived rating curve(s). UÊ Velocity measurement in floods is extremely difficult, although surface float tracking