Water use in alluvial fans refers to irrigation systems using the water resources in alluvial fans, mainly river floods and groundwater recharged by infiltration of rain or river water, to enhance the production of agricultural crops.
Background Alluvial fans, also called inland deltas, occur at the foot of mountain ranges and mark the presence of river floods. They contain considerable groundwater reservoirs that are replenished each year by infiltration of the water from the river branches into the usually permeable underground, thus creating rich aquifers. The mountainous areas usually receive more rainfall than the plains: they form a watershed and provide a source of water. In (semi)arid regions, therefore, alluvial fans are often used for irrigation of agricultural crops. The fans reveal much greenery in the harsh desert-like environment. Irrigation methods in alluvial fans differ according to the hydrological regime of the river, the shape of the fan, and the natural resources available to maintain human life.
Examples Khuzdar
The alluvial fans along the river plains near Khuzdar, Baluchistan, are fed by small water catchments in areas of relatively low mountains. The fans are relatively small, steep, and subject to flash floods ILRI, 1983. Modern interferences in traditional water resources in Baluchistan. Published in Annual Report 1982, p. 23-33. International Institute for Land Reclamation and Improvement (ILRI), Wageningen, The Netherlands. Public Domain. Reprinted in Water Research Journal (1983) 139, p. 53-60. Also reprinted in Water International 9 (1984), p.106- 111. Elsevier Sequoia, Amsterdam. On line:
Average annual rainfall in Baluchistan varies between 200 and 400 mm, depending on altitude, and the main part occurs in winter (November to March). Of old, in sloping lands, farmers constructed bunds along the contour lines to capture the surface runoff (Fig. K1). This method of water harvesting (locally called khuskaba) provided extra water for agricultural crops planted just up-slope of the bund, where the captured water would infiltrate into the soil and provide extra soil moisture to supplement the scarce rainfall.
In alluvial fans, the spate floods provided and additional source of water. The floods, diverted from the watercourses, were retained behind similar bunds employed in the khuskaba system (Fig. K2). The method of flood interception is locally called sailaba (Rod Kohi elsewhere in Pakistan, generally: spate irrigation). The system is combined with the tapping of groundwater from the aquifer by means of dug underground galleries, called karez or qanat (Fig. K3). The karezes make permanent agriculture possible (Fig. K3). Although the sailaba-and-qanat systems cover about 20% of the agricultural land, their production is more than 40% of the total.
It is a modern development to sink deep wells in the aquifer of the alluvial fan to exploit the groundwater more effectively than do the traditional karezes. The owners of the wells may be entrepreneurs from elsewhere and the original population runs the risk of losing their karez water when the wells draw the water table down to a deeper depth than that of the karezes so that these fall dry (Fig K4).
Garmsar
The fairly large alluvial fan of Garmsar, east of Tehran, is fed by the Hableh Rud river with an important catchment area in the high Alburz mountain range. The river carries a large amount of water during the rainy season, otherwise the discharge is low.ILRI, 2000. Irrigation, groundwater, drainage and soil salinity control in the alluvial fan of Garmsar. Consultancy assignment to the Food and Agriculture Organization (FAO) of the United Nations, International Institute for Land Reclamation and Improvement (ILRI), Wageningen, The Netherlands. Public Domain. On line:
The irrigation system for the Garmsar alluvial fan is quite well developed (Fig. G1, below), to the extent that lined canals have been made and a large belt-canal crosses the fan through its middle.
Roughly, the cropped area occupies 30% of the land each season, while 70% is left fallow. The winter crops are mainly wheat and barley, while the summer crops are cotton and melons. However, the planting of the new crops is done before harvesting the previous crops. Thus, there is a period of overlap during which 60% of the land is under crops. The fallow land is continuously rotated throughout the years, so that there exists no permanent fallow land, except along the fringes at the base of the fan where soil salinization occurs.
An estimated average annual water balance is shown in Fig. G2 (below). It is seen that the storage of irrigation losses in the aquifer plays an important role. In the dry season the groundwater is used for irrigation by pumping from deep wells. A cross-section of the groundwater situation is shown in Fig. G3 (below).
The water rights are expressed in sang, a measure of continuous flow of about 10 L/s, but in practice it varies from 10 to more than 15 L/s. The water is delivered to about 100 tertiary units (often a village), within which the water is distributed by 12-day rotations amongst the farmers who each are entitled to receive the authorized sangs for a fixed number of hours during each rotation period. The village communities are, at the same time, water-user associations who take care of the water-distribution within the tertiary unit and they maintain the tertiary canals.
At present, the distribution of surface irrigation water to the villages is determined by the Garmsar Water Authority on the basis of the water rights and verbal agreements and communications with the water users in the absence of a written manual. The authority also maintains the irrigation canals and structures. The structures are sometimes re-designed to adjust them to verbally communicated needs. The fair distribution of the irrigation water is not an easy job as the average annual river discharge is quite variable in the range of 5 to 20 m3/s (see graph at the right).
The deep tube-wells are privately owned. The drilling of wells is subject to license. Recently, the licensing has stopped for fear of over-exploitation of the aquifer. It appears that no operational rules are applied to the wells.
In the fringe lands, the water table is shallow because the discharge capacity of the aquifer diminishes here for two reasons: (1) the hydraulic gradient reduces where the sloping alluvial fan reaches the flat desert area, and (2) the thickness and hydraulic conductivity of the aquifer diminishes. The necessary drainage canals for watertable control at the fringes of the irrigation perimeter are not maintained by the water authority, but by the respective farmers groups. For the irrigation water, these groups depend (1) on occasional river floods too large to be handled by the irrigation system and that flow down to the fringe lands through the natural watercourses, (2) on spillage from the irrigation system, and (3) on deep wells.
To stabilize the agriculture in the fringe lands, which are threatened by soil salinization, a method of strip-cropping (Fig. G4) can be recommended for soil salinity control. This method uses irrigated strips next to permanently un-irrigated strips, whereby the salinization is directed to the un-irrigated strips. This concept is sometimes called sacrificial drainage.
Punata
The alluvial fan of Punata in the Valle Alto, east of Cochabamba, is fed by the Rio Paracaya river with a higher average discharge than the Hableh Rud, and consequently is fairly flat.R.J. Sevenhuijsen, R.J. Oosterbaan and K. Zijderveld, 1988. The Punata-Tiraque irrigation project near Cochabamba, Bolivia. International Institute for Land Reclamation and Improvement (ILRI), Wageningen, The Netherlands. Public Domain. On line:
The alluvial fan of Punata is found in the district of Cochabamba, Bolivia. The region of Punata, at the upper end of the Valle (valley) Alto, at about 2800 m altitude, has a summer rainfall of 400 to 450 mm starting in the second half of November end ending in March. Maize is here the most important food crop, followed by potatoes. Alfalfa is the dominant fodder crop, followed by maize straw. (Fig. P1). These crops could, of old, only be planted successfully because of the existence of additional water resources like runoff, floods, river base-flow and groundwater. In the winter months, crop growth is restricted due to the occurrence of night frosts, especially in June and July, and absence of rains.
The total rural population in Punata is estimated at 25 000. There are about 4000 families of which an estimated 3680 are farmer families. The farms are small. The average size is 1.3 ha of which 1 ha is cropped. The modal size of farm is smaller, about 0.7 ha.
The rainfall distribution in Punata is characterized by a wet season from December to March, a dry season from May to October, and transition months in April and November. The average yearly total is 428 mm (1966 to 1983, San Benito). The rainfall with a probability of exceedance of 75% (R75) on a year basis is 360 mm. Rainfall is not reliable: in the period from 1966 to 1983, the yearly total varied between 246 mm (1982/83) to 591 mm (1968/69).
The river floods during the rainy summer period can be used for irrigation by anyone who wants to. When the river flow recedes, the stream can only be used for rotational irrigation by those who are entitled to take part in it (this is locally called the mita system). By the month of May the river base-flow becomes strongly reduced, and a drought period sets in, lasting into November.
Irrigation is considered desirable to start the cropping season in August/September, so that an early harvest can be obtained. The early harvest has a high market value and reduces peak labor requirements. Further, the irrigation reduces the risk of crop failure and it permits diversification of agricultural produce. Nevertheless, there are some farming communities that have refrained in the past from the extra effort to obtain additional irrigation water and who seemed to be content with purely rain-fed cropping.
At a modest scale, irrigation from deep-wells is also practiced.
In order to satisfy the needs of the majority of the farmers who strongly wish to have additional irrigation water, the irrigation project Punata-Tiraque began to be developed from 1970 onwards. The project entailed the construction of a complicated system of dams and reservoirs up in the Andean mountains (Fig. P2).
The gross area of the Punata projects is estimated at 4600 ha, 90% of which can be used for agriculture or animal husbandry. About 1150 ha of this presently receive irrigation water, either surface water derived from the Laguna Robada or Lluska Kocha dam, or water pumped from the 16 deep wells in the project area (estimated at 350 ha). In addition there are a few hundred hectares that receive occasional water from mita irrigation (wild flooding).
The traditional irrigation method is based on handling large irrigation flows (golpes) per farm at large intervals. The intake structures in the Pucara Mayu river, at the place where it enters the alluvial fan of Punata, would alternately pass water from each of the reservoir systems (Laguna Robada and Lluska Kocha / Muyu Loma) and the natural mita water. The new system has been designed for smaller flows with shorter rotation intervals, but it works continuously for the whole area, so that there is no need anymore to separate the various sources of water. It covers a much larger area than the traditional system and it incorporates the associations of the mita systems (which may have partly the same members), the associations of tube-well systems (which may also have partly the same members) as well as the persons who had no previous water rights.