In floodwater harvesting systems (FloodWH), storm floods caused by runoff from mountainous catchments are channeled through diversions to bunded basins on cropped land. By transporting sediments from the catchments to croplands, these systems “grow” their own nutrient-rich soil. These systems play an important role in many arid and dry semi-arid regions worldwide – and the majority are traditional schemes. However, they are less widespread and less strongly promoted than micro and macrocatchment systems. An important reason is the more demanding planning at the watershed scale and the large volumes of water that must be managed – with the associated risks of serious erosion when flows breach barriers. Such systems depend on collective action between upstream and downstream land users and involve high labour input for annual maintenance. Despite the uncertainties regarding the timing and level of flooding, FloodWH technologies can sustain highly productive agricultural systems: centuries of tradition testify to this.

Water storage and purpose

Once floodwater is diverted to the cultivated area, it is stored in deep alluvial soils formed from the sediments deposited by previous floods. Annual crops, often under agroforestry systems, are then grown with the captured moisture. Alternatively, floodwater harvested within gullies/watercourses is stored in the sediment above structures and used to support the growth of trees, bushes or fodder crops.

Most common technologies

Flood recession farming and spate irrigation – where floodwater is deliberately diverted from the watercourse – are the most common amongst all FloodWH technologies. Water spreading weirs are known in parts of West Africa. Within streambed technologies such as jessour, tabias or “warping” are also well-known.

Floodwater harvesting can be further classified into:Floodwater diversion / off-streambed system, the channel water either floods over the river / channel bank onto adjacent plains (wild flooding) or is forced to leave its natural course and conveyed to nearby fields (Spate irrigation),Floodwater harvesting within streambed, the water flow is dammed and as a result, is ponded within the streambed. The water is forced to infiltrate and the accumulated soil water is used for agriculture.

Applicability

The diversion of floodwater is common in semi-arid and arid environments with extreme and highly variable rainfall regimes. It is often located where mountain catchments border plains: these downstream areas receive water from upstream catchments in the form of floods during heavy rainfall events. Resilience to climate variability an increase in flood events may provide more opportunities for FloodWH. However, if floods are too large, they can destroy diversion structures. Prolonged dry spells and droughts will increase insecurity because of the decreased number of floods. Further information about the applicability can be found here.

(Mekdaschi & Liniger 2013)

Read more about themain characteristics, benefits and disadvantagesof floodwater harvesting.