By Douglas Barnes
Click here for Part I
Click here for Part II
At the invitation of the Green Tree Foundation, I visited the town of Talupula in the drought-stricken Anantapur District in Andhra Pradesh. Once a dry tropical region, biotic pressures have changed the region into an arid landscape. Because of this, the Green Tree Foundation had me come in to design and implement water harvesting systems suitable for their area in an effort to assist with their regreening activities.
Having selected a site for our project, I mapped the boundaries of the site with a GPS unit, and assessed the site’s features and vegetation. Being ferrosols, the soil structure was generally pretty uniform from the surface down to as much as 8 metres deep. And being gravelly, it was not appropriate for dam construction. This meant we would stick to swales for this site. The cost of digging the swales was well within the total allotted budget for the project, so we next looked into choosing the best machine for the job.
While digging out swales with a bulldozer with a tilting blade can be a convenient way of making them, the soils were so hard that they would have created a near impossible situation for the dozer to handle. Add to that the fact that the nearest dozer would be 6 to 8 hours away, requiring a transport fee, and that they would only come out for more than 100 hours work, bulldozers were not an option anyway. We were left with the choice of a backhoe or a small excavator. The excavator had speed going for it, but availability was a problem (we would have access to it only one day a week). It also required transport to the site (increasing its cost) and was 35% more expensive than the backhoe. The backhoe looked to be the best choice by far.
Designing the water-harvesting system, I did not want the site to be plagued by undersized swales. For one thing, I wanted them to be able to hold a lot of water before excess would go over their spillways. Also, if they did not have gradual enough walls, they would be more prone to erosion. Over time, swales gradually fill in, too. With larger swales, they would be around longer.
The designer enjoying a fresh mango under a tamarind tree that saved him from heat stroke many times. Photo by Gangi Setty of the Green Tree Foundation.
As I wanted the swales to be around 1 metre from the bottom of the trench to the top of the mound, I designed the trench to be about 4 metres across (a little smaller on two of the swales) and about 4 metres across on the mound. The site was nearly devoid of vegetation, so to be on the safe side, I assumed 55% runoff, meaning a coefficient of runoff of 0.55. To determine the spacing of the swales, I used the formula
Spacing = Holding capacity per m ÷ (Runoff coefficient X Maximum rainfall in one event)
The volume of the swales per metre was to be around 1.7 cubic metres. The maximum rain in one large event in the area is 10 cm. From this, I calculated the approximate spacing for the spaces at 30 metres. Using the GPS, I was able to find the level for the second swale, 30m down from the top, then the 3rd level, 30 m down from there.
We needed to map out the contour sites, so we tracked down some local engineers with a dumpy level. It turns out that the engineers owed a friend of the landowner a favour, so they came out to the site for free and mapped out the contour points for 4 swales on 3 different contours. While I planned on mapping the site myself, their proficiency had them finishing the mapping in half the time it would have taken me. They put us ahead of schedule by one day.
The map of the site showing the swales in red and the level-sill spillways in yellow.
The swales were excavated without too much incident with the total excavation taking about 3 and a half days to move 600 cubic metres of earth. We didn’t have the backhoe do everything. We simply had it dig trenches 3 metres wide and 50 cm deep, placing the excavated earth on the downhill side of the trench. The rest we were leaving to be groomed by hand. At one point, we hit rock that would have taken hours for the backhoe to chip through. In such cases, it is simplest to try to go around either uphill or downhill around the rock. We opted to go uphill. Apart from that one little snag, the rest of the excavation went as quickly as one could expect considering the soil was nearly as hard as concrete.
We hired a team of ten labourers to groom the site. Because the soils were so hard, however, we had the backhoe come back and chip off the uphill edge of the trenches it dug to make the process faster. The first day of grooming was gruelling work for the work team. They had to rely on picks to be able to chip through the earth to smooth out the edges of the swale. Grooming the mound was much easier as the soil there was already broken up. In the end, I had hoped to get the mounds groomed to a more gentle slope while the workers were there, but time ran out before we could get everything as perfect as the ideal I held in mind. Still, the edges are not steep and erosion should not be a problem.
On the night before the final day of work, the heavens opened up and released a torrent on the site. Excited to see the swales in action, the landowner rushed out in the middle of the night to see them fill up with water that would otherwise have washed down the hill in an erosive flood. When I arrived on site the last day, the top swale and one of the lower swales were full of water due to the slower infiltration from their slightly higher clay contents. Already they were a home to some very happy frogs that, with the rains, had come out of hibernation.
The rains transformed the earth from a concrete-like surface to a soft and yielding one. This made the final grooming much, much easier. In order to help make the swales more durable, I had the work team put in compacted, level-sill spillways set at 90 cm from the bottom of the swales. With them in place, water can spill gently over the top in very heavy rains, greatly reducing the chance of erosion damaging the swales. The workers seemed to get a kick out of me inspecting the spillway with a site level and having them fix spots that were a few millimetres out. While it may seem fanatical, if the spillway is not dead-level, flowing water will concentrate in the lower spots. When it is concentrated, it moves faster, and when it moves faster, it has more erosive potential.
Workers compacting the soil to create a level-sill spillway to allow overflow without eroding the swale.
Since the monsoon season hit just as the project was completed, it started collecting water right away. Within three weeks of the completion of the swales, they had already captured and saved over half a million litres of water. The land owner was initially worried about the amount of land that the swales took up – land that would otherwise have been dedicated to the mango tree crop that is to go in later. But upon seeing the results of the swales in action, he knew they were the right thing to do.
I was very fortunate to have the agroforestry expertise of the Green Tree Foundation to assist in the selection of tree species from the site. The plan was to plant a windbreak crop and living fence consisting of Gliricidia sepium, Caesalpinia crista and Sapindus trifoliate. G. sepium is a fast-growing nitrogen fixer with medicinal properties. C. crista makes a good windbreak and has anti-malarial properties. Sapindus trifoliate, as the name suggests, is rich in saponins, meaning it makes a great soap. Its fruit, which resembles a date, is a valuable crop that fetches a good price on the local market. I have received word from the Green Tree Foundation that these windbreak trees have already been planted on site and are doing well. When the mango crops go in, the Green Tree Foundation will provide nitrogen-fixing support trees to assist in the growth of the mango trees.
Given the swales and the addition of the trees, I suspect that within 3-years time, springs will appear at the bottom of the hill below the site. With the site’s exposure next to the national highway and the growing notoriety of the farmer we worked with, it is hoped that our project will be replicated by others throughout the area. I have been invited back by my friends at the Green Tree Foundation to do more work in the area, and I look forward to the day when funding permits me to go there again and carry out more projects.