Rain Water Havesting (Part 1)

The benefits of Rain Water Harvesting are not just in preventing runoff, as was mentioned in my World Soil Day post. It is also an important tool for softening the effects of drought.
Helmreich and Horn examined the potential for rain water harvesting (RWH) for both agricultural and domestic uses. This post will examine their findings with respect to agriculture and see an example of agricultural RWH. Next post will examine domestic RHW and compare it to the methods used in agriculture.

Agricultural Uses

Helmreich and Horn Recommendations
The two types of agricultural RWH mentioned by Helmreich and Horn were (1) water stored in the soil where it fell and (2) water collected away from the farm and stored away from the crops.
The initial requirement for a good collection site is a surface material that allows runoff. Additionally the slope of the land must be great enough to cause the runoff to flow.
The collection point could be above, below, or within the ground. Underground was recommend over above-ground structures as it prevents evaporation. If the water was to be stored within the ground itself, then the soil must retain water well.
It was recommended that local labour and materials were used to assess areas to adapt and build the necessary infrastructure.

Implementation Example

Maswa, Tanzania is a semi-arid region where a lack of rain can prevent crop growth and trap farmers in poverty. The crops may miss out on the benefits of up to 80% of rain, because they can't absorb it fast enough. RWH allows farmers to replace low-profit, drought resistant crops, such as Sorghum (a cereal), with high-profit, thirsty crops, like rice and vegetables. A study by Hatibu et al. looks at the economic benefits of different times of RWH systems. The examined Microcatments, Macrocatchments, Macro catchments paired with road drainage, and Macro and Microcatchments paired with pond storage. These RWH were mainly made local labor and materials (lots of earth ditches) as recommended by Helmreich and Horn. While the roads are not locally funded, the RWH systems branching off from them are locally constructed.

Really quick term definitions:

Typical microcatchment set-up. Source: Ali et al.

Microcatchments- small enhancements in the topography (eg. trenches) to increase the amount of water going to plants; Type 1 of Helmreich and Horn, which they highlight as cheap and local

 

Due to the lack of other jobs, economic benefit
(in US$) is best measured with respect to land
area. Source: Hatibu et al.

Macrocatchments- similar principles as microcatchment but for diverting large pools of water that accumulate during storms; Type 2 collection with Type 1 storage of Helmreich and Horn

Marcocatchments paired with road drainage- this diverts the pre-existing large catchment of water (used to prevent flash floods on roads) to crops; Type 2 collection with Type 1 storage of Helmreich and Horn

Macro and Microcatchments paired with pond storage- allows farmer to store water away from the crops and water them when needed; Type 2 of Helmreich and Horn


Hatibu et al. found that that Marcocatchments paired with road drainage was the most profitable in terms of US$ per hectare (land area).  This finding exemplifies the dual benefit of flood and drought alleviation that RWH can provide. The pond storage might be improved by the underwater storage recommend by Helmreich and Horn. The pond's higher potential for evaporation may be way it is so much less efficient than the road drainage.