Floodwater the softest hardware
The Gareh Bygone Plain, a 6000-hectare sandy desert in southern Iran (28° 37´N, 53° 55´E, 1140m above mean sea level), annually receives 150mm of rain as opposed to 2860mm of Class A pan evaporation.
This plain had been covered with wild almond [Amygdalus scoparia (Spach)], wild pistachio [Pistacia atlantica (Desf.)], a close relative of Christs thorn [Ziziphus nummularia (Burm. F. Wight. And Walk.)] and a few associated species prior to the 1950s. The jungle provided a tranquil haven for herds of gazelles and flocks of hubara bustards, see see partridges, black-bellied sand grouses and many more bird species.
The advent of the farm tractor and Willys Jeep changed the picture completely. Systematic clear cutting of the scrubland for conversion to farms, and the wholesale hunting of the gazelles and hubara bustards while riding in Jeeps, left a denuded expanse reminiscent of a lunar landscape. But man had not ruined everything yet!
The arrival of diesel pumps in the Gareh Bygone Plain was the coup de grâce. The landlords dug wells and hit the water table at most at a depth of ten metres. As the wells increased in number the water table receded accordingly. The race had begun who could dig a deeper well and which brand of engine could pump more water to a greater height? Irrigated farms covered 168 hectares in 1967, the heyday of the Gareh Bygone Plain.
But there is a limit to every technology. Twenty metres was the maximum head that the available pumps could overcome. Most owners removed the pumping installations and left. Worse still, the receded water table made the qanats (underground water collection and conveyance galleries), which had sustained the three villages in the Gareh Bygone Plain for generations, dry, thus worthless. End of a civilization!
A drifting sand menace was the legacy of the ignorant souls buried by the inappropriate technology. How a single plant species could hold the sand in place and mitigate its erosion is utterly unbelievable. A rhizomatous sedge [Carex stenophylla (Wahl.)] forms a 10-cm armour immediately below the soil surface keeping the sand grains in place. Piercing this armour by ploughs exposes the loose sand to wind and water erosion. As long as the fields had a vegetative cover, or were under irrigation, sand did not drift. However, abandonment of the farms due to irrigation water shortages left the cropland defenceless against the winds which blow the year round, but mostly in the spring and summer months when the surface soil is bone dry.
The Gareh Bygone Plain is a microcosm of the desert of Iran and of other developing countries in the arid and semi-arid zones. Desertification and conversion to non-agricultural uses annually takes twelve to fifteen million hectares of farmland out of production. Man shall be out of fertile fields in about 100 years if this evil trend continues, assuming that we have already achieved zero population growth! To replace the abandoned fields, marginal lands are brought under the plough. More than half of the forests cleared annually are to replace the eroded agricultural land.
Fortunately, this vicious circle could be broken if there is a will to do so. Floodwater spreading for irrigation of drought-tolerant agronomic crops in floodplains devoid of shallow aquifers and the artificial recharge of groundwater on the debris cones, coarse alluvia and colluvial soils offer a practical and rather inexpensive method to reclaim the eroded land. This would help to build soil on the terrains where nature has reserved them for survival, and to make the wastelands bloom.
As the Gareh Bygone Plain is endowed with about 1.5km3 of alluvium, with a storage coefficient of ten per cent, we would emphasize the artificial recharge of groundwater activities. The floodwater spreading system, employed in the artificial recharge of groundwater system, may be used for irrigation also, with the result that a surface reservoir might replace the infiltration pond at the end of the system.
En masse city-ward migration had left the Gareh Bygone Plain to a few hardy souls who eked out a living from sixteen low yielding wells which could be operated from one to twenty hours per day. Saltwater intrusion into the aquifer compounded the water shortage problem. Long distance hauling of potable water, by carrying it inside goatskin bags on donkey back, became the womens and youngsters daily chore. Soil salinization was the natural outcome of irrigation with waters such as these.
We asked ourselves, What would our forefathers have done had they faced the same situation? The ancient Persians realized that they were living in a land of floods and droughts the so-called twin and alternative curses. They thought Why not store the abundant floodwaters underground and gradually deliver it through the qanats? The debris cones and alluvial fans of Khorasan in north-east Iran was the proving ground. Runoff farms, which cover more than 100,000 hectares in that province, are actually the sedimentation basins and infiltration ponds of the artificial recharge of groundwater schemes ringing the Great Iranian Desert.
We adapted the principle. Thus, a modern version of a very old technique was employed to rehabilitate the desert, bringing prosperity back to its devastated inhabitants proving to everybody, including the sceptical scientists and technologists, that desertification control through floodwater spreading was not a wild goose chase.
Our method is very simple. Floodwater is diverted into an inundation canal through an intake structure with, or without, a groin or an apron. A conveyor-spreader channel, which is a long, shallow stilling basin with a slope of three in ten thousand, whose excavated soil forms a bank on its upstream side, dissipates more than 90 per cent of the kinetic energy of the flow and spreads it as a shallow sheet of water on a very long front. Infiltration of some water into the soil and sedimentation of the coarse particles of the suspended load takes place simultaneously.
The surcharge of the first sedimentation basin enters the first level-silled channel, which is 100 to 300 metres downstream of the conveyor-spreader channel, through gaps installed in its bank. This process is repeated four to ten times until a rather clear water enters the infiltration ponds located at the far end of the artificial recharge of groundwater system. As a rule of thumb, we assign 100 litres per second per hectare for the debris cones, but the range is 50 to 250 litres per second per hectare.
As water is the most precious commodity in the desert, optimization of its consumption presents a very serious task for the water authorities. How do they assign shares for the different sectors of the economy whilst keeping in mind that whatever they do should be environmentally sound, financially viable, socially acceptable and politically prudent?
Carbon sequestering is one case in point. Eucalyptus camaldulensis (Dehnh.) grows well in the Gareh Bygone Plain. A few of the 14 year old trees are 23 metres tall, some with a diameter at breast height of 60cm. However, this species of tree is a prodigious water consumer. The six year old trees used 8096m3 per hectare of water during a five month period in the spring and summer of 1991. As their roots were in the phreatic zone (17.80 metres) when they were eleven years old, it is logical to assume that they consumed 20 to 25,000 m3 per hectare annually at that stage, which is very close to the potential evapotranspiration, and will consume more as they grow taller. Furthermore, it is safe to assume that each hectare of Eucalyptus camaldulensis can sequester five tons of carbon per year, both above and under ground.
We face a dilemma. To grow or not to grow Eucalyptus camaldulensis? If we economize our water consumption, each eight to ten year old tree replaces one adult person. The global warming issue dictates that we have to plant high yielding species and take carbon out of circulation for a specified number of years. Thus, this trade-off and conflict between water and carbon must be weighed against the potential benefits of conservation measures when assessing their effectiveness.
Floodwater spreading, as expected, has transformed a desert into verdant scenery. Some of the tangible benefits which were instrumental in reversing the migration towards the Gareh Bygone Plain are discussed below.
No adhesive may glue a dryland farmer to his fields better than water. More than 100 million m3 of water have reached the aquifer since 1983. According to the latest survey, 120 wells, eight of them with two operating pumps, irrigate 3482 hectares in the Gareh Bygone Plain. The average annual wheat yield of these fields is five metric tons per hectare or higher. Melon and watermelon bring the highest income because they are exported to the Persian Gulf Emirates. This has caused a ten-fold increase in the yearly income of the irrigation farmers. Although the cropland value has leaped over a thousand-fold, the knowledgeable new owner-operators may pay their loan instalments within two to three years.
Availability of freshwater is a great advantage in the deserts. The four villages which ring the artificial recharge of groundwater site are supplied by the wells directly recharged by good quality floodwater. Water salinity, as measured by electrical conductivity, has decreased from twenty to sixty-nine per cent.
Barley, the main feed grain, is a strategic stock for drought years when not enough forage is available and the herders have to undersell their sheep and goats.
Production of one ton of grain on one hectare, after only two years of floodwater spreading and relatively little sediment deposition, in the 1964/65 water year with a total rainfall of 153mm, was encouraging. This was repeated in the 1995/96 growing season when the average barley grain yield on a six hectare field in a sedimentation basin was 2355 kilograms per hectare.
The range in yield in the rain-fed farms in the same season was 570 to 1300 kilograms per hectare. The total amount of rainfall in this wet year was 313.5mm, more than twice the normal amount. At the going market price for grain and straw, a 250 kilograms per hectare increase in grain yield compensates the construction cost of an artificial recharge of groundwater system for one hectare in the very first year of operation.
In fifteen years of operation there have been four years without timely floods. This gives a twenty-seven per cent risk of complete failure in barley production with floodwater irrigation.
The annual dry matter yield of indigenous range plant species irrigated with floodwater ranged from 648 to 1603 kilograms per hectare per year with a mean of 996 kilograms per hectare per year in the 1991-96 period. Those for the control were 86-126 and 95 kilograms per hectare per year, respectively. As the mean forage yield of Atriplex lentiformis (Torr. Wats.) is 2.4 kilograms per plant per year, 1.5 tons of dry matter is produced annually at the stocking rate of 625 plants per hectare from the third year onwards.
Livestock browse the leaves and shoots of Acacia salicina (Lindl.), Acacia cyanophylla (Lindl.) and a little of Eucalyptus camaldulensis (Dehnh.). Therefore, it is safe to claim that the feed yield of the floodwater spreading system has increased more than thirty-fold.
The six species of eucalypt and the five species of acacia planted in the Gareh Bygone Plain provide pollen and nectar for the honey bees from March to October. Acacia salicina, which is in flower from September to March, spans the flowerless period of the other species. In fact, this is the only plant in the plain which is in flower in winter. We have not been able to fully determine the site potential for honey production yet, but three different example sites are worth mentioning: 700 beehives produced six tons of honey during a 45-day period in June and July of 1997; 100 beehives produced one ton of honey during a 30-day period; and, the average honey yield of 20 beehives kept at the station was 14.5 kilograms per beehive for a four-month period (March to June).
Inclusion of adapted honey-producing trees in afforestation programmes in southern Iran is on the agenda. This will revolutionize the bee-keeping industry.
A very important adjunct benefit of floodwater spreading, particularly for the artificial recharge of groundwater, is the covering of eroded land with a mantle of nutritious sediment.
As the floodwater is highly charged with livestock dung, dissolved nutrients and cations carried by the suspended load, the sediment is usually more fertile than the underlying eroded soil. Moreover, as the soils depth increases its available water capacity increases also; thus, it can supply water to the native or planted crops for a longer period. In fact, having deep soils is extremely advantageous in dry environments. We estimate that about one million m3 of fine-grained sediment has settled on about 1365 hectares of the floodwater spreading system since 1983.
To our amazement, the red river gum [Eucalyptus camaldulensis (Dehnh.)] has adapted itself to the arid environment of the Gareh Bygone Plain. The survival rate of eight year old trees planted at a three by three metre spacing was eighty-six per cent, and their average annual yield at the same age was 7.765m3 per hectare. The average height, diameter and basal area of these trees was 9.95m, 11.7cm and 11.00m2 per hectare, respectively.
In one example site this species was planted in one row, at seventeen stems per hectare, upstream of the channel banks at three metre spacing as wind breaks. The average annual yield at eight years old was 0.423m3 per hectare. The average height, diameter and basal area of these trees at eight years old were 14.00m, 18.0cm, and 0.43m2 per hectare, respectively.
Although the wood yield of other tree species has not been determined, from our experience with Acacia salicinia as a good fuelwood tree, it would compete successfully with the red river gum.
We have a concept of a Desert Utopia. What we are really striving for is to form a frame of mind for the desert-dwellers where they realize that their survival depends on respecting the natural resources. Water will replace money as the currency in our Desert Utopias. Water will be valued according to its availability, purity, temperature, subterranean flow rate and other factors. Water-use optimization is the key to the Desert Utopias continuing prosperity.
A hydro-agro-silvo-pastoral economy not necessarily in that order is the tenet of our Utopia. We have to do whatever possible to conserve the available water and keep it clean. Utilization of solar and wind energies are but only two of the mechanisms which could replace fossil fuel-produced electrical energy with all its pollution potential.
Sayyed Ahang Kowsar, Fars Research Center for Natural Resources and Animal Husbandry, PO Box 71365-458, Shiraz, Iran. Fax: +98-71-7205107. Email: email@example.com.