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{{Multiple issues, {{cleanup, date=April 2021, reason=Rephrase to remove journal-style arguments based on individual illustrations, condense excessive technical detail{{primary sources, date=April 2021{{overly detailed, date=April 2021 Water use in alluvial fans refers to
irrigation Irrigation (also referred to as watering of plants) is the practice of applying controlled amounts of water to land to help grow crops, landscape plants, and lawns. Irrigation has been a key aspect of agriculture for over 5,000 years and has bee ...
systems using the
water resources Water resources are natural resources of water that are potentially useful for humans, for example as a source of drinking water supply or irrigation water. These resources can be either Fresh water, freshwater from natural sources, or water produ ...
in
alluvial fan An alluvial fan is an accumulation of sediments that fans outwards from a concentrated source of sediments, such as a narrow canyon emerging from an escarpment. They are characteristic of mountainous terrain in arid to Semi-arid climate, semiar ...
s, mainly
river flood A flood is an overflow of water ( or rarely other fluids) that submerges land that is usually dry. In the sense of "flowing water", the word may also be applied to the inflow of the tide. Floods are of significant concern in agriculture, civi ...
s and
groundwater Groundwater is the water present beneath Earth's surface in rock and Pore space in soil, soil pore spaces and in the fractures of stratum, rock formations. About 30 percent of all readily available fresh water in the world is groundwater. A unit ...
recharged by infiltration of rain or river water, to enhance the production of agricultural crops.


Background

Alluvial fan An alluvial fan is an accumulation of sediments that fans outwards from a concentrated source of sediments, such as a narrow canyon emerging from an escarpment. They are characteristic of mountainous terrain in arid to Semi-arid climate, semiar ...
s, also called inland deltas, occur at the foot of mountain ranges and mark the presence of river floods. They contain considerable
groundwater Groundwater is the water present beneath Earth's surface in rock and Pore space in soil, soil pore spaces and in the fractures of stratum, rock formations. About 30 percent of all readily available fresh water in the world is groundwater. A unit ...
reservoirs that are replenished each year by infiltration of the water from the river branches into the usually permeable underground, thus creating rich
aquifer An aquifer is an underground layer of water-bearing material, consisting of permeability (Earth sciences), permeable or fractured rock, or of unconsolidated materials (gravel, sand, or silt). Aquifers vary greatly in their characteristics. The s ...
s. The mountainous areas usually receive more rainfall than the plains: they form a
watershed Watershed may refer to: Hydrology * Drainage divide, the line that separates neighbouring drainage basins * Drainage basin, an area of land where surface water converges (North American usage) Music * Watershed Music Festival, an annual country ...
and provide a source of water. In (semi)arid regions, therefore, alluvial fans are often used for
irrigation Irrigation (also referred to as watering of plants) is the practice of applying controlled amounts of water to land to help grow crops, landscape plants, and lawns. Irrigation has been a key aspect of agriculture for over 5,000 years and has bee ...
of agricultural crops. The fans reveal much greenery in the harsh
desert A desert is a landscape where little precipitation occurs and, consequently, living conditions create unique biomes and ecosystems. The lack of vegetation exposes the unprotected surface of the ground to denudation. About one-third of the la ...
-like environment. Irrigation methods in alluvial fans differ according to the
hydrological Hydrology () is the scientific study of the movement, distribution, and management of water on Earth and other planets, including the water cycle, water resources, and drainage basin sustainability. A practitioner of hydrology is called a hydro ...
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 Khuzdar ( Balochi, , ) also known as ''Hozdar'', is a city and the administrative headquarters of Khuzdar District in Balochistan province of Pakistan. Khuzdar is surrounded by mountainous region of the southwestern portion of the country near ...
,
Baluchistan Balochistan ( ; , ), also spelled as Baluchistan or Baluchestan, is a historical region in West and South Asia, located in the Iranian plateau's far southeast and bordering the Indian Plate and the Arabian Sea coastline. This arid region of de ...
, are fed by small water catchments in areas of relatively low mountains. The fans are relatively small, steep, and subject to flash floods 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 Rainwater harvesting (RWH) is the collection and storage of rain, rather than allowing it to run off. Rainwater is collected from a roof-like surface and redirected to a tank, cistern, deep pit (well, shaft, or borehole), aquifer, or a reservoir w ...
(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 Groundwater is the water present beneath Earth's surface in rock and Pore space in soil, soil pore spaces and in the fractures of stratum, rock formations. About 30 percent of all readily available fresh water in the world is groundwater. A unit ...
from the
aquifer An aquifer is an underground layer of water-bearing material, consisting of permeability (Earth sciences), permeable or fractured rock, or of unconsolidated materials (gravel, sand, or silt). Aquifers vary greatly in their characteristics. The s ...
by means of dug underground galleries, called ''karez'' or
qanat A qanāt () or kārīz () is a water supply system that was developed in ancient Iran for the purpose of transporting usable water to the surface from an aquifer or a well through an underground aqueduct. Originating approximately 3,000 years ...
(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 the ''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). Image:KhuzdarBund.JPG, Fig. K1. An earthen bund to retain runoff for crop production Image:KhuzdarAgri.JPG, Fig. K3. Permanent irrigation and agriculture based on the ''karez'' system in a small valley in Baluchistan


Garmsar

The fairly large alluvial fan of Garmsar, east of
Tehran Tehran (; , ''Tehrân'') is the capital and largest city of Iran. It is the capital of Tehran province, and the administrative center for Tehran County and its Central District (Tehran County), Central District. With a population of around 9. ...
, is fed by the Hableh Rud river with an important catchment area in the high
Alburz The Alborz ( ) range, also spelled as Alburz, Elburz or Elborz, is a mountain range in northern Iran that stretches from the border of Azerbaijan along the western and entire southern coast of the Caspian Sea and finally runs northeast and merge ...
mountain range. The river carries a large amount of water during the rainy season, otherwise the discharge is low. 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 Soil, also commonly referred to as earth, is a mixture of organic matter, minerals, gases, water, and organisms that together support the life of plants and soil organisms. Some scientific definitions distinguish dirt from ''soil'' by restrict ...
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 An aquifer is an underground layer of water-bearing material, consisting of permeability (Earth sciences), permeable or fractured rock, or of unconsolidated materials (gravel, sand, or silt). Aquifers vary greatly in their characteristics. The s ...
plays an important role. In the
dry season The dry season is a yearly period of low rainfall, especially in the tropics. The weather in the tropics is dominated by the tropical rain belt, which moves from the northern to the southern tropics and back over the course of the year. The t ...
the
groundwater Groundwater is the water present beneath Earth's surface in rock and Pore space in soil, soil pore spaces and in the fractures of stratum, rock formations. About 30 percent of all readily available fresh water in the world is groundwater. A unit ...
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 Water right in water law is the right of a user to use water from a water source, e.g., a river, stream, pond or source of groundwater. In areas with plentiful water and few users, such systems are generally not complicated or contentious. In o ...
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 A license (American English) or licence (Commonwealth English) is an official permission or permit to do, use, or own something (as well as the document of that permission or permit). A license is granted by a party (licensor) to another part ...
. Recently, the licensing has stopped for fear of
over-exploitation Overexploitation, also called overharvesting or ecological overshoot, refers to harvesting a renewable resource to the point of diminishing returns. Continued overexploitation can lead to the destruction of the resource, as it will be unable t ...
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 In geotechnical engineering, watertable control is the practice of controlling the height of the water table by drainage. Its main applications are in agricultural land (to improve the crop yield using agricultural drainage systems) and in cities ...
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 Soil, also commonly referred to as earth, is a mixture of organic matter, minerals, gases, water, and organisms that together support the life of plants and soil organisms. Some scientific definitions distinguish dirt from ''soil'' by restrict ...
, a method of strip-cropping (Fig. G4) can be recommended for
soil salinity control Soil salinity control refers to controlling the process and progress of soil salinity to prevent soil degradation by salination and land reclamation, reclamation of already salty (saline) soils. Soil reclamation is also known as soil improvement ...
. 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''. Image:GarmsarMap.JPG, Fig. G1. Map of the Garmsar area showing irrigation systems Image:GarmsarAquifer.JPG, Fig. G3. Cross-section through the aquifer showing the geohydrologic conditions


Punata

The alluvial fan of
Punata Punata is the capital of Punata Province and Punata Municipality in Cochabamba Department, Bolivia Bolivia, officially the Plurinational State of Bolivia, is a landlocked country located in central South America. The country features dive ...
in the ''Valle Alto'', east of
Cochabamba Cochabamba (; ) is a city and municipality in central Bolivia in a valley in the Andes mountain range. It is the capital (political), capital of the Cochabamba Department and the list of cities in Bolivia, fourth largest city in Bolivia, with ...
, is fed by the Rio Paracaya river with a higher average discharge than the Hableh Rud, and consequently is fairly flat. The alluvial fan of Punata is found in the district of
Cochabamba Cochabamba (; ) is a city and municipality in central Bolivia in a valley in the Andes mountain range. It is the capital (political), capital of the Cochabamba Department and the list of cities in Bolivia, fourth largest city in Bolivia, with ...
,
Bolivia Bolivia, officially the Plurinational State of Bolivia, is a landlocked country located in central South America. The country features diverse geography, including vast Amazonian plains, tropical lowlands, mountains, the Gran Chaco Province, w ...
. 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 Maize (; ''Zea mays''), also known as corn in North American English, is a tall stout grass that produces cereal grain. It was domesticated by indigenous peoples in southern Mexico about 9,000 years ago from wild teosinte. Native American ...
is here the most important food crop, followed by
potatoes The potato () is a starchy tuberous vegetable native to the Americas that is consumed as a staple food in many parts of the world. Potatoes are underground stem tubers of the plant ''Solanum tuberosum'', a perennial in the nightshade famil ...
.
Alfalfa Alfalfa () (''Medicago sativa''), also called lucerne, is a perennial plant, perennial flowering plant in the legume family Fabaceae. It is cultivated as an important forage crop in many countries around the world. It is used for grazing, hay, ...
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 Groundwater is the water present beneath Earth's surface in rock and Pore space in soil, soil pore spaces and in the fractures of stratum, rock formations. About 30 percent of all readily available fresh water in the world is groundwater. A unit ...
. 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
farm A farm (also called an agricultural holding) is an area of land that is devoted primarily to agricultural processes with the primary objective of producing food and other crops; it is the basic facility in food production. The name is used fo ...
s 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 Rain is a form of precipitation where water droplets that have condensed from atmospheric water vapor fall under gravity. Rain is a major component of the water cycle and is responsible for depositing most of the fresh water on the Earth. ...
distribution in Punata is characterized by a wet season from December to March, a
dry season The dry season is a yearly period of low rainfall, especially in the tropics. The weather in the tropics is dominated by the tropical rain belt, which moves from the northern to the southern tropics and back over the course of the year. The t ...
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 Probability is a branch of mathematics and statistics concerning events and numerical descriptions of how likely they are to occur. The probability of an event is a number between 0 and 1; the larger the probability, the more likely an e ...
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 Irrigation (also referred to as watering of plants) is the practice of applying controlled amounts of water to land to help grow crops, landscape plants, and lawns. Irrigation has been a key aspect of agriculture for over 5,000 years and has bee ...
is considered desirable to start the cropping season in August/September, so that an early
harvest Harvesting is the process of collecting plants, animals, or fish (as well as fungi) as food, especially the process of gathering mature crops, and "the harvest" also refers to the collected crops. Reaping is the cutting of grain or pulses fo ...
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 Agriculture encompasses crop and livestock production, aquaculture, and forestry for food and non-food products. Agriculture was a key factor in the rise of sedentary human civilization, whereby farming of domesticated species created f ...
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
dam A dam is a barrier that stops or restricts the flow of surface water or underground streams. Reservoirs created by dams not only suppress floods but also provide water for activities such as irrigation, human consumption, industrial use, aqua ...
s and reservoirs up in the
Andean mountains The Andes ( ), Andes Mountains or Andean Mountain Range (; ) are the longest continental mountain range in the world, forming a continuous highland along the western edge of South America. The range is long and wide (widest between 18°S ...
(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. Hence, the new irrigation system makes it necessary to replace the traditional water rights by a totally new set of rights (and duties). In addition, the farmers will have to get used to new water distribution methods and new field irrigation techniques. Because the new irrigation zones do not correspond to the boundaries of the existing, scattered, ''Comité’s de Riego'' (Fig. P3), not only the water management but also the organizational structure will have to be adjusted to the new situation. Image:PunataCropping.JPG, Fig. P1. Typical cropping calendar of an irrigated farm in the Punata area Image:PunataTransmis.JPG, Fig. P4. The permeability and transmissivity of the aquifer diminishes towards the end of the alluvial fan


Okavango

The Okavango inland delta, near Maun, receives an enormous amount of river inflow from
Angola Angola, officially the Republic of Angola, is a country on the west-Central Africa, central coast of Southern Africa. It is the second-largest Portuguese-speaking world, Portuguese-speaking (Lusophone) country in both total area and List of c ...
. Hence, the fan is so large and flat that it is rather called a delta. It takes six months for the peak inflow at the apex to reach the base of the delta.R.J. Oosterbaan, L.F. Kortenhorst, and L.H.Sprey, 1987. ''Flood-recession cropping in the ''molapo's'' of the Okavango Inland Delta, Botswana''. Published in Annual Report 1986, p. 8 – 19. Public Domain. International Institute for Land Reclamation and Improvement (ILRI), Wageningen, The Netherlands. Public domain. On line

/ref>


Features

The inland Okavango Delta in northwest
Botswana Botswana, officially the Republic of Botswana, is a landlocked country in Southern Africa. Botswana is topographically flat, with approximately 70 percent of its territory part of the Kalahari Desert. It is bordered by South Africa to the sou ...
is hand-shaped, with the fingers spread (see map). The Okavango River, which originates in Angola, enters the delta at its apex. On the average, the river carries about 10 000 million m3 of water a year into the delta. The flow rate is high in the months of March and April (about 1000 m3 /s on the average), but varying from year to year between 500 and 1500 m3/s and low in November (100 to 200 m3 /s). The large volume of water spreading over the delta is almost fully absorbed in permanent and seasonal swamps (the latter are called ''molapo’s'') before is slowly infiltrates or evaporates. There is rich swamp vegetation, which creates an ideal environment for numerous kinds of animals. The rich fauna finds its habitat on and between the thousands of islands between the swamps. The little water that exceeds the retention capacity of the
marsh In ecology, a marsh is a wetland that is dominated by herbaceous plants rather than by woody plants.Keddy, P.A. 2010. Wetland Ecology: Principles and Conservation (2nd edition). Cambridge University Press, Cambridge, UK. 497 p More in genera ...
y wilderness drains from July to November through the fingers of the giant hand. Hence, it takes almost six months before the peak discharge of the Okavango River manifests itself at the base of the delta. Here, the water meets a barrier: the Thamalakane Fault Line (see map), beyond which the
Kalahari The Kalahari Desert is a large semiarid sandy savanna in Southern Africa covering including much of Botswana as well as parts of Namibia and South Africa. It is not to be confused with the Angolan, Namibian, and South African Namib coastal d ...
sands rise up 10 m. At the foot of the fault, the Thamalakane River collects the water (which is not more than 5% of the total inflow) and carries it almost without gradient to the Boteti River, which flows through a breach in the fault line. Eventually, the remaining waters
evaporate Evaporation is a type of vaporization that occurs on the surface of a liquid as it changes into the gas phase. A high concentration of the evaporating substance in the surrounding gas significantly slows down evaporation, such as when hum ...
in the
Makgadikgadi Pan The Makgadikgadi Pan ( Tswana pronunciation ), a salt pan situated in the middle of the dry savanna of north-eastern Botswana, is one of the largest salt flats in the world. The pan is all that remains of the formerly enormous Lake Makgadi ...
s, more than 200 km to the east. Although the annual
rainfall Rain is a form of precipitation where water droplets that have condensed from atmospheric water vapor fall under gravity. Rain is a major component of the water cycle and is responsible for depositing most of the fresh water on the Earth. ...
is relatively low (an average of 500 mm, the greater part of which falls from December to March), it contributes a volume of water to the Delta equaling half of the Okavango inflow. The annual rainfall and its yearly distribution are equally erratic as the regime of the river. The Okavango River transports a large amount of
sand Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is usually defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural ...
s and other
sediments Sediment is a solid material that is transported to a new location where it is deposited. It occurs naturally and, through the processes of weathering and erosion, is broken down and subsequently sediment transport, transported by the action of ...
into the delta. Their mass is about 2 million tons a year.
Salts In chemistry, a salt or ionic compound is a chemical compound consisting of an assembly of positively charged ions ( cations) and negatively charged ions (anions), which results in a compound with no net electric charge (electrically neutral). ...
also enter the Delta, but they do so in a dissolved form. The salt concentration of the water is some 200 mg/L, which is very low. The total weight of incoming salts is thus about 2 million tons per year. The sediments and the salts imported by the Okavango River settle in the delta. Together with the
vegetation Vegetation is an assemblage of plants and the ground cover they provide. It is a general term, without specific reference to particular Taxon, taxa, life forms, structure, Spatial ecology, spatial extent, or any other specific Botany, botanic ...
the sediments build up resistances to surface-water flow. As a result, the main
watercourse A stream is a continuous body of water, body of surface water Current (stream), flowing within the stream bed, bed and bank (geography), banks of a channel (geography), channel. Depending on its location or certain characteristics, a strea ...
s have in the past swayed from thumb to little finger to and fro, as is common in alluvial fans.
Tectonic movement Plate tectonics (, ) is the scientific theory that the Earth's lithosphere comprises a number of large tectonic plates, which have been slowly moving since 3–4 billion years ago. The model builds on the concept of , an idea developed durin ...
s have also contributed to this phenomenon. At present the middle finger, from which the Boro River stems, provides the major thoroughfare. Many of the islands in the delta have a garland of riparian trees along their borders, but in the middle they are bare: symptoms of salt accumulation (see photo of the delta). The Kalahari Desert cooperates with the Okavango River to form the predominantly sandy soils of the delta. The desert uses the vector wind to deposit its share of fine sand. The geophysical characteristics of the Okavango Delta have led to a low population density, so that the natural situation has scarcely been disrupted by mankind. Also, the population was more interested in
hunting Hunting is the Human activity, human practice of seeking, pursuing, capturing, and killing wildlife or feral animals. The most common reasons for humans to hunt are to obtain the animal's body for meat and useful animal products (fur/hide (sk ...
and
cattle breeding Animal husbandry is the branch of agriculture concerned with animals that are raised for meat, fibre, milk, or other products. It includes day-to-day care, management, production, nutrition, selective breeding, and the raising of livestock ...
than in
food crop A crop is a plant that can be grown and harvested extensively for profit or subsistence. In other words, a crop is a plant or plant product that is grown for a specific purpose such as food, fibre, or fuel. When plants of the same species ar ...
production, so that agricultural developments were limited. The
arable land Arable land (from the , "able to be ploughed") is any land capable of being ploughed and used to grow crops.''Oxford English Dictionary'', "arable, ''adj''. and ''n.''" Oxford University Press (Oxford), 2013. Alternatively, for the purposes of a ...
s in the south-eastern fringes of the delta, that become dry after the floods recede (these are locally known as molapo's), often have a sandy topsoil. In depressions, the topsoil may be thin or missing altogether, exposing a heavy
clay Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolinite, ). Most pure clay minerals are white or light-coloured, but natural clays show a variety of colours from impuriti ...
soil.


Developments

In 1978/79, after four years of high and prolonged floods had made ''molapo'' farming impossible, a severe
drought A drought is a period of drier-than-normal conditions.Douville, H., K. Raghavan, J. Renwick, R.P. Allan, P.A. Arias, M. Barlow, R. Cerezo-Mota, A. Cherchi, T.Y. Gan, J. Gergis, D.  Jiang, A.  Khan, W.  Pokam Mba, D.  Rosenfeld, J. Tierney, ...
coincided with an outbreak of
foot and mouth disease Foot-and-mouth disease (FMD) or hoof-and-mouth disease (HMD) is an infectious and sometimes fatal viral disease that primarily affects even-toed ungulates, including domestic and wild bovids. The virus causes a high fever lasting two to six d ...
, leaving the local population in a state of emergency. This resulted in two important undertakings: *The first took place from 1979 to 1981 when, as part of a drought-relief program (Food for Work), FAO organized labor-intensive works to rehabilitate the flood-control bunds that the local population had built to protect their crops against inundations from rainstorms. Some new bunds were also built. *The second was the construction of the “Buffalo-fence” (see map), which separates the outer fringes of the delta from its interior to prevent the spread of cattle diseases – especially
foot and mouth disease Foot-and-mouth disease (FMD) or hoof-and-mouth disease (HMD) is an infectious and sometimes fatal viral disease that primarily affects even-toed ungulates, including domestic and wild bovids. The virus causes a high fever lasting two to six d ...
. Completed in 1983, this fence has increased the importance of the ''molapo’s'' outside it, and for the following reasons. Especially in the years when rains start late, the new grass flush in the ''molapo’s'' after flood-recession presents about the only source of
fodder Fodder (), also called provender (), is any agriculture, agricultural foodstuff used specifically to feed domesticated livestock, such as cattle, domestic rabbit, rabbits, sheep, horses, chickens and pigs. "Fodder" refers particularly to food ...
in the region, also for the
herd A herd is a social group of certain animals of the same species, either wild or domestic. The form of collective animal behavior associated with this is called '' herding''. These animals are known as gregarious animals. The term ''herd'' ...
s in the wide surroundings of the delta. With the ''molapo’s'' inside the fence closed to cattle
grazing In agriculture, grazing is a method of animal husbandry whereby domestic livestock are allowed outdoors to free range (roam around) and consume wild vegetations in order to feed conversion ratio, convert the otherwise indigestible (by human diges ...
, the grazing intensity in the ''molapo’s'' outside the fence has increased. The Molapo Development Projects (MDP) became operational in December 1983. The project aimed at increasing crop production in pilot areas by protecting those areas against prolonged high floods by flood-control bunds with entrance gates that could be closed (see photo). When enough floodwater has entered the molapo, the gate can be closed and the water recedes under influence of
evaporation Evaporation is a type of vaporization that occurs on the Interface (chemistry), surface of a liquid as it changes into the gas phase. A high concentration of the evaporating substance in the surrounding gas significantly slows down evapora ...
and infiltration into the soil and cropping may start when the outside water level is still high. This was a response to the high and prolonged flooding in the years 1974–1978, when ''molapo'' cropping was largely impossible. In more recent years, however, inadequate floods appeared to present an equally severe constraint to satisfactory crop production. It was therefore decided to focus also on improved, more stable crop production under fully rain-dependent conditions. The hydrograph figure shows that crop production has not been possible in 60% of the hitherto recorded years because of the prolonged high floods. The figure also shows how flood-control measures can bring about a timely recession of the water level in the molapo. After the flood has been permitted to enter the bunded molapo, the
sluice gate A sluice ( ) is a water channel containing a sluice gate, a type of lock to manage the water flow and water level. There are various types of sluice gates, including flap sluice gates and fan gates. Different depths are calculated when design s ...
s are closed. Recession of the water in the bunded molapo then begins under the influence of evaporation and infiltration, and allows a timely planting of the crop (in October or November). The crops use the residual
soil moisture Soil moisture is the water content of the soil. It can be expressed in terms of volume or weight. Soil moisture measurement can be based on ''in situ'' probes (e.g., capacitance probes, neutron probes) or remote sensing methods. Water that enters ...
(about 100 mm) till the onset of the
rainy season The rainy season is the time of year when most of a region's average annual rainfall occurs. Rainy Season may also refer to: * ''Rainy Season'' (short story), a 1989 short horror story by Stephen King * "Rainy Season", a 2018 song by Monni * '' ...
at the end of November or the beginning of December. Thus the growing season is prolonged, the moisture availability is increased, and crop production is enhanced. However, the success of the flood-control measures on the crop performance still depends to a great extent on the amount and distribution of the rainfall.


References

Land management Hydrology Irrigation Hydraulic engineering