This post
will highlight some of the suggestions put forward by Rockstrom and Falkenmark
(2015) in retaining and storing green water. Rainwater harvesting is the 'small- scale concentration, collection, storage and use of rainwater runoff for productive purposes' (Kahinda et al 2007: 1050).
Green water
can be stored or retained in various ways. Firstly, the run off can be
collected such as in ponds or tanks (Figure
1). This is ideal in the light of climate change when rainfall events are
likely to get heavier and more infrequent. When a rainfall event is high
intensity it means that crops can get damaged or soils worn away of nutrients.
Therefore, collecting this water will not only reduce this impact but it will
also mean there will be water to use as the rainfall gets more infrequent.
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Figure 1- Rainwater harvesting using a water tank.
Another way
of harvesting green water is by using Fanya juu terraces like those in
Machakos, Kenya (Graham 1991). This technique
is used to improve arable land where soil erosion has damaged the productivity
of crop fields. The soil is mounted upwards from a ditch to form an earth
embankment. Several of these are made across one field. The rainwater is
conserved between the mounts and this keeps rain in the area and keeps the soil
in the field. Therefore, there is better moisture for the crops and nutrients
in the soil are not worn away. Although it is labour intensive, it is low-
skilled and does not require much training.
The final
suggestion by Rockstrom and Falkenmark (2015) is using ditches to channel
runoff into fields. I, however, think this is very problematic. The Hjulstrom
curve (1935*) (figure 2), amongst
many other things, shows that fine particles are easily transported whilst
coarser particles are less easily transported from the area of erosion to the
sedimentation site. Thus, the channel water is likely to erode the ditch or silt
up with coarse material. There would need to be regular maintenance of the
channel which can be expensive and be a source of conflict.
Figure 2- The Hjulstrom curve.
Consequently,
Rockstrom and Falkenmark (2015) have some promising suggestions but as shown
above some can be problematic.
*Please note Hjulstrom's thesis, from which the graph was first used, is unavailable online.
*Please note Hjulstrom's thesis, from which the graph was first used, is unavailable online.
Nice to see the frequency of blogposts increasing. Good use of peer-reviewed literature is made and it is the inclusion of figures is excellent. I like the inclusion of the Hjulstrom Curve and the critical comments on sediment transport - nicely done. Rainwater harvesting in all its myriad of forms is an interesting focal area as it pertains to agriculture and I encourage you to delve more deeply here. Be good to see greater interaction on your blog so perhaps encourage some fellow students in your thematic area to comment and offer to do so in return.
ReplyDeleteGood to see a blog discussing green water storage as it does seem like the simplest solution. My concern with this method though is the sheer size of tanks required to store 6 - 8 month worth of water. Did you come across how big the tanks would need to be in your research? It'd be interesting to know.
ReplyDeleteGood question!
DeleteI didn't come across this but it would be interesting to know, so will let you know.
Something to keep in mind though, I would suggest they use a combination of rainwater harvesting techniques simultaneously to overcome issues like the one you just mentioned. Also, with the unreliability of water, (in the light of climate change making rainfall events less frequent and more intense), maybe using rainwater harvesting along with other options like groundwater would be more reliable for them.