Runoff and Floodwater Farming in Middle East and American Southwest

Floodwater is the water that runs through valleys, streets, arroyos and wadis after a rain storm. Sparse vegetation, little soil development, soil crusts, and low infiltration/absorption contribute to fast peaking flows in arid regions. Storms lead to floods that may continue for minutes, hours or days depending on the rainfall. Flood depths of 5-10 meters may occur in extreme events. Despite the challenges, residents in arid and semi-arid lands have developed a wide range of strategies to hold and capture critically needed water. In many cases, floodwater has been the primary source of drinking water for people and livestock, water to irrigate trees and crops, and to replenish groundwater.

Floodwater also carries organic materials and fine sediment that can be captured to improve soil fertility, soil tilth and structure. Tohono O’odham families in the American Southwest (SW) sought out places where moist, nitrogen rich litter had accumulated beneath mesquite trees and would dig up the top soil and spread it on farm fields. Nitrogen fixation may take place at 5-8 meters depth so these deep roots can provide little competition for shallow-rooted crop plants grown nearby.

Floodwaters in these desert ecosystems can carry large amounts of rodent dung, leaves from nitrogen fixing trees and shrubs, litter, and twigs. Enough material may come to floodwater irrigated fields in these floods to add an inch of rich soil and organic matter a year. Over hundreds of years the deposits grow quite deep. Studies suggest sediment deposits up to 30 m or more deep in the Marib area resulted from irrigation.

Floodwater can be destructive when flows are so big they overwhelm the infrastructure, damage homes and farms, and put people at risk. The ancient city of Petra was hit by a massive flood in the 4th or 5th century. This was a rare, catastrophic event. Water rushing down wadis would have created a fast moving 5-8 meter deep flood rushing down the main street. It would have left boulders, heaps of gravel and pebbles and a road covered with 2 to 5 meters of sediment. Capturing more water higher up in the landscape with rock lines, terraces, dams and cisterns reduces the risk from floods, but would not prevent these mega floods.

To better understand the options for runoff agricuture here is a review of some of the imaginative and effective techniques developed to use runoff and flood water in the Mediterranean and North Africa (MENA) and the American Southwest (SW).

Experimentation and experience over more than 5,000 years led to remarkable systems that enabled people to live well even in the most arid environments. These systems can be built and maintained by local people using available tools. Two major goals are to hold the water back so it doesn’t run off and to concentrate the water so that even the most arid areas people have enough water to drink and sufficient water to grow a crop. As climate change increases the severity of droughts and floods these techniques may be rediscovered and rebuilt. They will also prove useful in new areas.

Water retention

The first goal is capturing and holding water so it can sink into the soil or fill a pond or cistern. Techniques range from a simple line of rocks on the contour to complex sets of dams in the wadis sized to capture just enough water to maximize soil water holding capacity in irrigated fields.

The oldest versions of run-off farming probably consisted of water spreading and/or small rock or brush check dams to slow and collect run-off. Small rock dams are found throughout the SW. One or two rainfall events with runoff can be sufficient to grow a crop. In areas with very limited rainfall the people of the SW would plant seed only when the soil was wetted. Much work was done while the floods were in progress. An everyday sight during showers was the irrigators at work with hoes or sticks, or even with their hands. Cultivars were selected that allowed for deep burial of the seeds, up to 15 cm or more. This provides soil moisture for a longer period of root growth and enables the crop to withstand another surface flow flooding event.

Terraces retain runoff and transform slopes into flat areas that are easier to plant and harvest. Most people are familiar with the terraced rice paddies of Asia, but equally impressive terraces for agroforestry, grains and crops can be found in the MENA countries and American SW. The trees and shrubs on terraces provide food, fodder, fuel and wood while helping to stabilize the terraces and, in many cases, providing a source of income. They help conserve soil and protect it from runoff and erosion. Agricultural terraces require regular maintenance and if it lapses, it can lead to the collapse of retaining walls accompanied by increased soil erosion.

Microcatchments of various kinds are also effective. These can be crescent shaped, rectangular, or square. Some have ridges on all four sides. Microcatchments have been used for millennia in the MidEast, Africa, and the Americas.

Microcatchments are built at low gradients. The area inside the microcatchment is steeper with flow leading to the lowest spot. About 10% of the rain that falls on the catchment may flow. The desired area can be calculated with an understanding of the crop, the weather, and the soil. In the Negev the catchments were 17-30 times the planted plot. This could give an augmented rainfall equivalent to 300-500 mm with just 100 mm of precipitation. This is enough to grow most crops.

Soil pits (zai) are smaller but can be effective. Pitting improves water infiltration and retention, reduces evaporation, and increases surface storage and the time available for infiltration to occur. Pits capture rainfall directly and get a minor boost from runoff. Zai can collect up to 25% or more run-off. Not as much as water as the microcatchments, but still of value. They increase surface water storage and water capture, allowing water to seep deeper into the soil. In one study moisture penetration reached 61 cm on pitted soil but only 12.7 cm on unpitted soil.

Systems that collect runoff in larger catchments such as hillsides with long slopes may be  called macro-catchments. The larger open reservoirs of MENA may be called hafirs, tabias and limans. In arid Tunisia, the tabia system is a traditional macro-catchment with a runoff area that occupies two-thirds of the slope and is traditionally used for grazing; with one to five cropped plots within U-shaped soil banks arranged in a cascade in the third downstream area. These run-on areas accumulate and store the occasional runoff. In Southern Sudan, hafirs provide water to livestock, agriculture, humans and, to some extent, wildlife.

Small field plots were leveled and terraced to ensure the efficient distribution of water as well as the conservation of both water and soil. Research with various fruit and fodder trees in the Negev has shown that during and after a rainfall event significant amounts of soil moisture in the topsoil between tree rows on terrace fields were not fully utilized and could be used for an intercrop.

In Northern Kenya runoff capture irrigation made it possible to double crop. Normally, local Turkana farmers won’t risk a second crop during the short rains in August/September. But with rainwater capture the second crops were more reliable and could be further improved with mulching and tree pruning. A variant of these are the hillside conduit systems found in the Negev. There ridges and channels run down slope collecting water to augment wadi flow to crop fields.

Water for people and animals can be stored in cisterns. A remarkable variety and number of cisterns in Petra were filled by runoff from rocks acting as the catchment. Small grooves or ridges would direct water into a sediment basin and then into the cistern. Rainwater can also be captured and stored in bigger cisterns or ponds. The most impressive cisterns are those designed to capture floodwater during flash floods. The Nahal Zin cistern is filled only after the flow in the arroyo is 1.5 meters deep. The full cistern can hold 1,400 cubic meters of water. A large cistern in Resafa, Syria was filled with floods from a wadi west of the city to a full capacity of 18,000 cubic meters. These floodwater filled cisterns would need to have sediment removed periodically.

Dams used by ancient communities were typically small and used to hold water for drinking and animals. For example, the transhumant pastoralists of the Sheeb region in Eritrea build small water diversion structures. Larger systems of wadi bed floodwater harvesting involved a series of stepped dams built across wadi beds. These shallow ponds would partially or completely fill with flood water and recharge the soil moisture so a crop could be grown. A flood event we monitored in the SW desert recharged the soil moisture to 7 meters and it remained high for months. Permanent and temporary dams of various kinds were used to divert water from wadis onto adjacent fields. This could be small scale or large.

The Maʾrib Dam in Yemen, the largest, was built around two thousand years ago to regulate the waters of the Wadi Sadd. It was about 550 meters long built of fine stone-and-masonry construction, with sluice gates to control the flow of water. It irrigated more than 1,600 hectares and was used and repaired for hundreds of years. A Sabaean inscription from 449 CE records acquisition of, “14,000 camels, 200,000 sheep (seems high), 217,000 pounds of flour as well as 630 camel loads of beverages” to supply the needs of workers mobilized to repair the Ma’rib dam.

Flood waters are generally diverted rather than impounded. Dams may divert water into an acequia or distribution ditch. The diversion dams can be permanent or temporary, rebuilt each year with brush and trees and rocks. These could be large permanent structures as well. Floodwaters in Wadi Mahra were deflected by large barrages and channeled into primary canals tens of meters wide. Waters were slowed before being divided into secondary and tertiary channels on their way to fields. High floods may damage diversion dams but they are easily rebuilt. Diversion dams can also be designed to shunt excess floodwater to a bypass to minimize damage to farm fields, diverters, dams and cisterns.

Floodwater farming is still practiced in arid and semi-arid lands. Studies in the Middle East, Africa, the American SW, Mexico and many other areas have reconsidered the history and value of runoff farming. Here are a few highlights.

Yemen

Yemen has used a wide range of water capture strategies, including extensive terraces, to capture floods. Floodwater (spate, sayl) irrigation was once common in Yemen. and involved flooding agricultural plots surrounded by field ridges (bunds, soum). These field bunds are typically 50-60 cm high. The farm land is served by diversion canals up to several kilometers long. The irrigation of most of the agricultural lands in Yemen’s southern and eastern governorates use floodwater farming. Using technologies like those at Ma’rib the neighboring kingdoms of Ma’in, Qatabān, Awsan and Hadramawt captured floodwater runoff from Yemen’s highlands.

Syria

Studies at Hayt al-Suad and Jubabat al-Juruf yielded wheat, barley and other domesticated crops dated to the late 4th to mid-third millennium BCE. This would be some of the regions first agricultural terraces. The ancient floodwater harvesting system in Resafa, Syria was reliable and consisted of extensive embankments, a dam and cisterns to provide water for a city that has no perennial water sources.

Jordan

The rainwater systems around Petra have been studied in some detail. Much of the water captured by these systems would have been for people and animals. The amount needed for a family of six with donkeys, camels, sheep or goats would be about 18 m3 year. So for the city of 20,000 people the annual demand might be as high as 90,000 m3. Runoff farming on flatter areas and hill slopes grew food for local use and export. Slopes were terraced or had contour rock/ridge lines. These rock line/ridges were either level or sloped to direct water to cisterns, tanks or fields.

Israel

The ancient agricultural systems in the Negev Highlands were erected in several phases beginning in the 3rd millennium BCE, and were used and re-built until the Early Islamic Period (7th-11th centuries CE). The main construction elements in runoff desert agriculture were the agricultural terraces that controlled water flow in the dry wadis. The intensity of use in Israel was illustrated at Ramat Bet Shemesh where most of the area is covered by agricultural terraces. Thousands of ancient terraces in the Negev desert show that agriculture was based on the capture and use of runoff and floodwater. Comprehensive studies of floodwater farms in the Negev highlight how well these systems worked. The most striking feature are the multiple wadi terrace dams. These were typically spaced from 12-15 meters apart. The heights were set to capture enough water to fully  recharge the soil. The irrigation farms developed complex designs with flood bypass and diversion channels.

To increase water capture the farmers deliberately cleared the stones off the slopes, smoothed the surface, and exposed finer soil to facilitate the formation of a self-sealing crust that would increase runoff to farm fields. Typical farm units of 0.5-5 hectares were associated with 10-150 hectares of sloping watershed. The ratio of run-off-contributing catchment to runoff-receiving crop land varied from 20:1 to 30:1. Each plot would receive enough water to produce a crop most years.

The American Southwest

The Anasazi culture of the SW developed from about 100 CE to 1400 CE. They had highly developed rain water capture management systems with check dams, reservoirs, cisterns, and canals with diverters. The Morefield Reservoir persisted across centuries., Detailed soil analysis revealed that its ancient engineers operated it for 350 years with 14 forest fires and 21 periods of high water flow. The community had to work diligently to harvest this water and maintain access to it. Runoff used as water supply carried a high volume of sediment that required frequently dredging to maintain adequate capacity.

In the Tohono O’Odham (formerly known as Papago) territory of southern Arizona the flood run-off of the mountain areas is gathered in streams with well defined channels which on reaching the great undissected alluvial basins spread out into sheets. The place where this spreading occurs is called by the Ak-Chin or arroyo mouth. Similar practices were found in other tribes, including the Navajo.

The method of planting instead of plowing furrows reduces erosion. Each seed spot is opened with a stick. Deep planting assures a strong root system capable of resistance to drought and to some surface washing by flood water. Formerly, as many as ten fields were continuous in one ephemeral watercourse. A local community of men shared the responsibility of keeping the watercourse and associated ditches clean and free from brush. They also repaired damage to the water control structures. By collecting water off mesquite dominated watersheds and allowing nitrogen-rich mulch to flow down into the fields they harvested nutrients as well. When Gary Nabhan worked with the last generation of flood-water farmers they were part of a 4,000 year unbroken chain. Soil fertility in their fields was equal to modern Corn Belt corn fields that are annually fertilized with nitrogen.

The Hohokam lived in the Phoenix Basin along the Gila and Salt Rivers, in southern Arizona along the Santa Cruz and San Pedro Rivers, and north on the Lower Verde River and along the New and Agua Fria Rivers. The Hohokam villages were remarkably stable. Unlike ancient pueblo towns, which often were abandoned, some Hohokam villages were continuously occupied for up to 1,500 years or more. The Hohokam (300 BCE-1450 CE) were gone by the time the Spanish arrived about 1600 CE, but the remains of their irrigation systems are still visible. They included main and distribution canals and field laterals. They built more than 483 kilometers of major canals and over 1,126 km of distribution canals. Unfortunately the dynamic nature of these rivers has ensured that we do not have any detail about diversion dams, headgates, or intake portions of canals. We can’t say how frequently these structures were destroyed or required repair as a result of larger floods.

The waffle gardens of the Zuni people are a combination of ridge and strip collectors that look much like a waffle. The ridges are packed smooth and serve as walkways and water runoff areas. The soil berms surround each square planting area. The depressions catch and hold water close to the plant’s roots. Grid gardens are similar, with larger square or rectangular grids bordered with rocks. These stones may have been the base for mud walls as early explorers saw them in the Zuni gardens. Larger versions of grid gardens have been used in the Southwest and also for dryland farming throughout the world.

The Hopi and Tohono O’odham of the SW plant just below the “mouths of washes” where storm waters naturally spread out and wet the soil. With these ak-chin fields farmers harvested both rainwater and nutrients to grow corn, squash, beans, and melons. Some of the garden terraces at Paaqavi (Bacavi) have been in use since, approximately 1200 CE.

Mexico

Rainwater harvesting has been developed and practiced since before the Spanish conquest. The Mayan and Aztec cultures captured and distributed rainwater using channels for both drinking and irrigating their crops during the dry season. The acequia culture also developed in Mexico.

Lessons for the Future

The ancient masters of runoff and flood agriculture can provide lessons for farmers today. Increases in atmospheric heat retaining gasses are expected to produce an increase in mean global surface temperatures of between 1.5°C and 4.5°C. Warmer temperatures with climate change will increase evaporation, reducing surface water and drying out soils and vegetation.

The Southwestern US has already seen a decrease in annual precipitation since the beginning of the 20th century, and that trend is expected to continue. Changes in the amount and distribution (or seasonality) of precipitation have also been predicted, but models are less capable of future changes in seasonal or annual precipitation. Dry places will become drier and available soil moisture in summer may decrease by 15 to 20 percent. This creates the need for expanded water capture and storage before and during drought years. Climate change will also alter the timing of water availability. Warmer winter temperatures are causing less precipitation to fall as snow, so runoff or flood irrigation can be hit with decreased snowpack or glacial melt.

Climate change with increased drought, reduced stream flow, and the breakdown of infrastructure may make traditional floodwater farming a necessity in some areas. These changes will also challenge rainwater farmers. Water harvesting doesn’t work if there is no rain. More severe storms will harm dams, canals, and diverters. Hail and high winds can damage plants. Erosion can remove topsoil and deplete soil nutrients.

The other lessons from these runoff farmers is the importance of community and cooperation. Keeping runoff and floodwater systems functioning took timely and significant work to repair damage and remove sediment. Working together is essential. A shovel of dirt in the right place, at the right time, might avert a disaster.

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