4.1 Introduction
4.2 Crop production systems
4.3 Examples on how to calculate the ratio C: Ca
4.4 Systems for trees
4.4 Systems for rangeland and fodder
Each WH system consists of a catchment (collection) and a cultivated (concentration) area. The relationship between the two, in terms of size, determines by what factor the rainfall will be "multiplied". For an appropriate design of a system, it is recommended to determine the ratio between catchment (C) and cultivated (CA) area.
Many successful water harvesting systems have been established by merely estimating the ratio between catchment and cultivated area. This may indeed be the only possible approach where basic data such as rainfall, runoff and crop water requirements are not known. However, calculation of the ratio will certainly result in a more efficient and effective system provided the basic data are available and accurate.
Nevertheless, it should be noted that calculations are always based on parameters with high variability. Rainfall and runoff are characteristically erratic in regions where WH is practised. It is, therefore, sometimes necessary to modify an original design in the light of experience, and often it will be useful to incorporate safety measures, such as cut-off drains, to avoid damage in years when rainfall exceeds the design rainfall.
The calculation of C:CA ratio is primarily useful for WH systems where crops are intended to be grown. This will be discussed first.
Figure 14. Catchment-cultivated area ratio - The principle
The calculation of the catchment: cultivated area ratio is based on the concept that the design must comply with the rule:
WATER HARVESTED = EXTRA WATER REQUIRED |
The amount of water harvested from the catchment area is a function of the amount of runoff created by the rainfall on the area. This runoff, for a defined time scale, is calculated by multiplying a "design" rainfall with a runoff coefficient. As not all runoff can be efficiently utilized (because of deep percolation losses, etc.) it must be additionally multiplied with an efficiency factor.
WATER HARVESTED = CATCHMENT AREA X DESIGN RAINFALL X RUNOFF COEFFICIENT X EFFICIENCY FACTOR |
The amount of water required is obtained by multiplying the size of the cultivated area with the net crop water requirements which is the total water requirement less the assumed "design" rainfall.
EXTRA WATER REQUIRED = CULTIVATED AREA X (CROP WATER REQUIREMENT - DESIGN RAINFALL) |
By substitution in our original equation
WATER HARVESTED = EXTRA WATER REQUIRED |
we obtain:
CATCHMENT AREA X DESIGN RAINFALL X RUNOFF COEFF. X EFF. FACTOR = CULTIVATED AREA X (CROP WATER REQUIREMENT - DESIGN RAINFALL) |
If this formula is rearranged we finally obtain:
_{} |
Crop Water Requirement
Crop water requirement depends on the kind of crop and the climate of the place where it is grown. Estimates as given in Chapter 2 should be used when precise data are not available.
Design Rainfall
The design rainfall is set by calculations or estimates (see Chapter 3). It is the amount of seasonal rain at which, or above which, the system is designed to provide enough runoff to meet the crop water requirement. If the rainfall is below the "design rainfall," there is a risk of crop failure due to moisture stress. When rainfall is above the "design", then runoff will be in surplus and may overtop the bunds.
Design rainfall is calculated at a certain probability of occurrence. If, for example, it is set at a 67% probability, it will be met or exceeded (on average) in two years out of three and the harvested rain will satisfy the crop water requirements also in two out of three years.
A conservative design would be based on a higher probability (which means a lower design rainfall), in order to make the system more "reliable" and thus to meet the crop water requirements more frequently. However the associated risk would be a more frequent flooding of the system in years where rainfall exceeds the design rainfall.
Runoff Coefficient
This is the proportion of rainfall which flows along the ground as surface runoff. It depends amongst other factors on the degree of slope, soil type, vegetation cover, antecedent soil moisture, rainfall intensity and duration. The coefficient ranges usually between 0.1 and 0.5. When measured data are not available, the coefficient may be estimated from experience. However, this method should be avoided whenever possible (see Chapter 3).
Efficiency Factor
This factor takes into account the inefficiency of uneven distribution of the water within the field as well as losses due to evaporation and deep percolation. Where the cultivated area is levelled and smooth the efficiency is higher. Microcatchment systems have higher efficiencies as water is usually less deeply ponded. Selection of the factor is left to the discretion of the designer based on his experience and of the actual technique selected. Normally the factor ranges between 0.5 and 0.75.
a. Example One
Climate: Arid
RWH System: External Catchment (e.g. trapezoidal bunds)
Crop Millet:
- Crop Water Requirement for Millet (total growing season) = 475 mm (low because rapid maturity)- Design Rainfall (growing season) == 250 mm (at a probability level of P = 67%)
- Runoff Coefficient (seasonal) = 0.25 (low due to relatively long catchment and low slope)
- Efficiency Factor = 0.5 (general estimate for long slope technique)
_{}
i.e.: The catchment area must be 7.2 times larger than the cultivated area (in other words, the catchment: cultivated area ratio is 7.2:1)
Comment: The ratio is high, but the system is designed for a dry area with a low runoff coefficient assumed.
b. Example Two:
Climate: Semi-Arid
RWH System: External Catchment (e.g. trapezoidal bunds)
Crop: 110 day Sorghum
- Crop Water Requirement = 525 mm
- Design Rainfall = 375 mm (P = 67%)
- Runoff Coefficient = 0.25
- Efficiency Factor = 0.5
_{}
i.e: The catchment area must be 3.2 times larger than the cultivated area. In other words, the catchment: cultivated area ratio is 3.2:1.
Comment: A ratio of approximately 3:1 is common and widely appropriate.
c. Example Three:
Climate: Semi-Arid
RWH System: Microcatchment (e.g. contour ridges)
Crop: 110 day Sorghum
- Crop Water Requirement = 525 mm- Design Rainfall = 310 mm (set at a probability level of P = 75% to give more reliability)
- Runoff Coefficient == 0.5 (reflecting the high proportion of runoff from very short catchments)
- Efficiency Factor == 0.75 (reflecting the greater efficiency of short slope catchments)
_{}
i.e. The catchment area must be approximately twice as large as the cultivated area.
Comment: Ratios are always lower for microcatchment systems due to a higher efficiency of water use and a higher runoff coefficient. Using a design rainfall of 67% probability (i.e. a less reliable system) would have even reduced the ratio to 1:1.
The ratio between catchment and cultivated area is difficult to determine for systems where trees are intended to be grown. As already discussed, only rough estimates are available for the water requirements of the indigenous, multi-purpose species commonly planted in WH systems. Furthermore, trees are almost exclusively grown in microcatchment systems where it is difficult to determine which proportion of the total area is actually exploited by the root zone bearing in mind the different stages of root development over the years before a seedling has grown into a mature tree.
Figure 15. Microcatchment system (Negarim microcatchment) for trees
In view of the above, it is therefore considered sufficient to estimate only the total size of the microcatchment (MC), that is the catchment and cultivated area (infiltration pit) together, for which the following formula can be used:
_{}
where:
MC = total size of microcatchment (m^{2})
RA = area exploited by root system (m^{2})
WR = water requirement (annual) (mm)
DR = design rainfall (annual) (mm)
K = runoff coefficient (annual)
EFF = efficiency factor
As a rule of thumb, it can be assumed that the area to be exploited by the root system is equal to the area of the canopy of the tree.
Example:
Semi-arid area, fruit tree grown in Negarim microcatchment
Annual water requirement (WR) = 1000 mm
Annual design rainfall (DR) = 350 mm
Canopy of mature tree (RA) = 10 m^{2}
Runoff coefficient (K) = 0.5
Efficiency factor (EFF) = 0.5
Total size MC = 10 x {(1000-350)/(350 x 0.5 x 0.5)} = 84m^{2}
Table 15 shows some examples of catchment and cultivated area sizes for several species. The range in dimensions is remarkable.
As a rule of thumb, for multipurpose trees in the arid/semi-arid regions, the size of the microcatchment per tree (catchment and cultivated area together) should range between 10 and 100 square metres, depending on the aridity of the area and the species grown. Flexibility can be introduced by planting more than one tree seedling within the system and removing surplus seedlings at a later stage if necessary.
In most cases it is not necessary to calculate the ratio C:CA for systems implementing fodder production and/or rangeland rehabilitation. As a general guideline, a ratio of 2:1 to 3:1 for microcatchments (which are normally used) is appropriate.
Table 15 - DIMENSIONS OF CATCHMENTS AND CULTIVATED AREAS FOR TREE MICROCATCHMENTS
Species |
Country |
Catchment Area (m2) |
Cultivated Area (m2) |
Source |
Ziziphus mauritiana |
Rajastan, India |
31.5-72 |
36 |
Sharma et al. (1986) |
Pomegranate |
Negev, Israel |
160 |
16 |
Shanan & Tadmore (1979) |
Almonds |
Negev, Israel |
250 |
10 |
Ben-Asher (1988) |
Fodder/Fuelwood spp. |
Baringo, Kenya |
10-20* |
Critchley & Reij (1989) | |
Fodder spp. |
Turkana, Kenya |
93.75* |
6.25** |
Barrow (in Rocheleu et al. (1988) |
Fuelwood/multi-purpose spp. |
Guesselbodi Forest, Niger |
64* |
Critchley & Reij (1989) | |
Fuelwood/multi-purpose spp. |
Keita Valley, Niger |
12 |
1.8 |
Critchley & Reij (1989) |
* No breakdown given between catchment and cultivated area/infiltration pit.
* * In a number of cases two trees planted within the same system.