Rain Water Harvesting on a 11.9 Hectare site " Dulga polyana"

I am currently working as part of a design team for a 11.9 hectare farming project here in Bulgaria. The project will consist of an  Organic Cherry Orchard designed by  The Research Institute of Organic Agriculture (FiBL)  and a Forest Garden with Cherry featuring as the dominant Upper Canopy specie. My role is to design a Forest Garden area as well as looking into the rainwater harvesting potential of the site.

View from the top of the watershed looking South

The immediate challenge is to provide an irrigation solution that will meet the water needs for the whole site. Its a difficult site in that although flanked by two streams to the east and west and the river Chervishtitsa  to South , the land is like a huge wedge rising up above the flanking streams. Relying solely on water from the streams is not feasible or desirable, as water levels are very low in the dry months. Interception will inevitably disrupt the existing ecosystems and expensive pumping will be necessary to get the water uphill.

There are over 5000 Cherry trees to be planted on this land and all will need irrigating, particularly in the early years. We estimated the water needs of the site to be 3300-3500m3 per year based on cultivating approx 7 ha of the 11.9 ha site.

Annual Average Rainfall data for the region along with favorable topography indicates good potential for harvesting rainwater and storing it on the land in one or more reservoirs. My mission is to establish the practicalities of this.  

  

A View from the South East / Boundaries marked with green line 

My main focus during my first two day field trip was to get an impression of how water is interacting with the site. I walked the perimeter of the site marking on a GPS any signs of water erosion i.e rills and gullies and looked for patches of vegetation that stood out in the landscape such as dry or lush patches. I logged the elevation of key points throughout the site and tracked the source of perennial water flows i.e streams and rivers around the land. The aim was to build a picture of how the rainfall would move across the land , where it would settle, where it would drain and identify where best to intercept and store this water. 

Once back to the computer I uploaded the GPS data to google earth and proceeded to sketch out my observations from the field onto the satellite image. I included erosion rills and gullies, elevation readings, slopes, current drainage channels etc. All this info on the map helps to build a vision of rainwater catchment potential of the site. With rainwater catchment the idea is to prevent water from draining off the site (causing erosion in this case) and divert it to an area where it can be stored. The ideal area to store the water should be the highest point on the land in order for the water reserve to be able to irrigate the largest percentage of crops via gravity.

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The next step was to know what quantity of water is available to store. To work this out I needed to identify the watershed, i.e the area of surrounding land that contributes to a surface flow of water from rainfall events.

Watershed above The Site

Topographic maps are a great way of indicating the slope of the land. OpenStreetMap provides contour mapping for the world with 10m contour intervals, i.e each contour line is a difference of 10m elevation   from each other. This contour mapping can also overlay onto google earth. You can also adjust the appearance of the terrain on Google earth  if you would like the elevation to appear more pronounced in your views by modifying the 'Elevation Exaggeration value'. The default value is set to 1, but you can set it to any value from 1 to 3, including decimal points. Go to "Tools"  than to "Options" and you will see 'Elevation Exaggeration value' field under the Terrain section. This is great for getting a general overview of the terrain.   

1.5000 Contour Map of the site. The narrow contour lines indicate that the land slopes sharply  into the flanking streams 

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Calculating Potential Rainwater Harvesting from Surface Runoff Annually

The rainwater catchment area above the site is approx 59624m2. This figure only includes the area of land  that slopes towards the eastern and western streams and provides a conservative estimate of land that will contribute surface flow to the drainage channels that provide input to the reservoirs

Taking a low estimate of the average annual rainfall for the region at 527mm. This indicates that the area can be expected to receive approx 527mm of rain per year
By multiplying annual average rainfall (527mm) with the area of the watershed, (59624m2), we can estimate how much rain can be expected to fall on the watershed in an average year.

59624m2 x 527mm = 31421848 L per annum

Taking into account the absorption and evaporation rates of this water into the soil I have used a conservative Runoff Coefficient Value of 0.1 in order to obtain the expected amount of surface runoff that can be expected to drain from the watershed.  

59624 x 527 x 0.1 = 3142184.8 L per annum

3142184.8 L = 3142m3

The next step is to establish a means to harness that water and store it on the landscape. 

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