5 minute readRainwater harvesting (RWH): intro

This is the first subject I would like to continue working on now: rainwater harvesting, or RWH. An example of RWH is the mamateo from the pre-Incan people in the Andes. There are of course many different types of RWH, and literature has summed them up nicely for me. In this post I will show various criteria and methods which are for RWH. In a later post I will do the actual processing.

What is RAINWATER harvesting?

Water harvesting is not new, it is thousands of years old, but its potential remains largely unknown, unacknowledged and
unappreciated1. So what is it? The principle is to capture potentially damaging rainfall runoff and translate this into plant growth or water supply.

Zai planting pits are designed to collect rainwater and to conserve nutrients in order to improve crop production and food security.
Zai planting pits are designed to collect rainwater and to conserve nutrients in order to improve crop production and food security. (Source)

 

And where is it useful

Of all the agriculture worldwide, more than 80% depends on rainfall, with Sub-Saharan Africa and Latin America reaching 90%. The areas that depend on rainfed agriculture are usually also the areas where the majority of the rural poor live, often also areas where food insecurity is hight. So there is great potential for these areas to become at least self-sufficient in food production.

Rainfed farming in arid, semi-arid, sub-humid and even humid regions are full of challenges. There is a high variety in rainfall quantity and timing, which can lead to droughts and floods. It is clear that if the amount of rainfall is less than the required amount of water for the crops, the yield will be lower than the potential yield.

This is further strengthened by climate change (isn’t everything?), where predictions estimate yield reductions with increasing temperature.

Not only is the water not used optimally, land degradation from soil erosion (wind and water) and poor management lead to low water use efficiency. Water use efficiency decreases because of evaporation of water on the surface, surface runoff and deep percolation (where the water is out of reach for the crop). Mind, these are complex systems, so infiltrated water may re-appear more downstream, and evaporated water can fall as rain somewhere else. So the water use efficiency depends on where the boundaries are also.

There is much more to read about effects water harvesting in relation to irrigation, which are found here, but I won’t go into it any further (maybe in a later post). I will go into how GIS can help!

Techniques and criteria

Ponds and pans, dams, terracing, percolation tanks, and Nala bunds (a nice story of how these check dams restored life in a rural village can be found here) are the most common types of RWH techniques in Arid and semi-arid regions2. Although there are many techniques, they all have the same components:Ā  a catchment or collection area, the runoff conveyance system, a storage part and an application area. If you want to know more about the designs and their pro’s and con’s, I highly recommend you to look on WOCAT’s website about this.

Rainwater harvesting: Nala bund (a check dam) design for catching surface water runoff on a slope
Rainwater harvesting: Nala bund design for catching surface water runoff (Source)

There are as many definitions of RWH as techniques, but they all have in common that the collection and management of runoff water for other uses is important. The main goal of rainwater harvesting is of course to increase water availability. The success of rainwater harvesting depends a lot on suitable site identification and their technical design3. This is where WOCAT has less information about, although it does give information on the optimal range of natural environments of a technique (annual rainfall, slope, soil depth, etc.). In this sense,

Rainwater harvesting site selection

Conducting field surveys is a common method for site selection in small areas but for large areas this method requires a lot of time. Imagine a mountainous area, with areas that are difficult to reach. With the developments of computers in the ’80s, geographic information systems (GIS) and remote sensing (RS) also grew, making it easier to survey large areas (although fieldwork is still needed to confirm findings).

The following six ‘themes’ can be useful in determining a place for the RWH technique:

  • climate
  • hydrology
  • topography
  • agronomy
  • soils, and
  • socio-economics

of which the first five are easy to derive from maps (once you have them šŸ˜€ ). Socio-economics is more tricky, but also includes distance to roads for example.

However, the required specific information depends on the rainwater harvesting technique (for example slope ranges). This is where GIS and RS are strong. All these (weighted) properties can be overlaid with each other, and the a suitability map rolls out. But as we will see in later posts, there are different ways to overlay these different maps.

Next step

We will find out what the different site selection methods yield in later posts, once I’ve found a study area to play around with šŸ˜‰ . I will then look for a nice location for a rainwater harvesting technique, based on its environmental criteria from WOCAT.

PS: here is the source of the cover image4

1.
Mekdaschi Studer R, Liniger H. Water Harvesting: Guidelines to Good Practice. Bern: Geographica Bernensia; 2013.
2.
Y. Oweis T, Prinz D, Y. Hachum A. Rainwater Harvesting for Agriculture in the Dry Areas. CRC Press; 2012.
3.
Al-Adamat R, AlAyyash S, Al-Amoush H, et al. The Combination of Indigenous Knowledge and Geo-Informatics for Water Harvesting Siting in the Jordanian Badia. J. 2012;04(04):366-376. doi:10.4236/jgis.2012.44042
4.
Singh LK, Jha MK, Chowdary VM. Multi-criteria analysis and GIS modeling for identifying prospective water harvesting and artificial recharge sites for sustainable water supply. J. 2017;142:1436-1456. doi:10.1016/j.jclepro.2016.11.163

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