7.8 What amount of effluent or area is required for irrigation?
When calculating the amount of water required for irrigation, the input of a skilled agronomist is crucial. This expert will evaluate the appropriateness of crops based on the quality and availability of wastewater.
Once the selection of crops is finalised, the agronomist can determine the monthly water demand for irrigation using the following formula:
| Parameter | Description |
|---|---|
| DW | Total monthly demand of water for agriculture in m3/(ha x month) |
| 10 | The conversion factor from mm/month to m3/(ha x month) |
| ET0 | Potential evapotranspiration in mm/month In general, the authority provides these data or can be calculated by the Penman equation. |
| AC | The area of each crop in ha |
| kC | The crop coefficient (kc) value of each crop (ratio of ET0 to actual ET) is determined through a combination of field observations, measurements, and research studies. It is a dimensionless factor that can vary not only between different crops but also within the same crop based on various factors. |
| ETa | Overall efficiency of water used in the conveyance system, in the irrigation (irrigation method) application method, and so on. It does not necessarily include infiltration losses (which can be treated as a sensitivity parameter, for example, for 1mm infiltration, 5 mm, and so on) |
The provided equation is a simple way to estimate the area needed for hydraulic purposes. However, it does not take into consideration factors such as monthly precipitation (P), which can, at times, fulfil irrigation requirements during rainy periods.
Crop coefficient (kc) can vary not only between different crops but also within the same crop species based on factors such as local climate, soil conditions, and agricultural practices. They are used to adjust the reference crop evapotranspiration to estimate the water needs of specific crops during different stages of growth. For instance, banana palms have a kc of 0.5 during the initial stage of the first year and 1.2 during the middle stage of the second year. Additional storage area is also necessary as wastewater may not be required for irrigation throughout the year.
Effluent reuse in irrigation is a cost-effective way of providing essential nutrients, such as nitrogen, phosphorus, and potassium (NPK), as natural fertilisers to crops. However, the amount of nutrients that effluent can provide to crops depends on several factors such as effluent nutrient concentrations, irrigation methods, and crop types. Therefore, estimating the percentage of fertiliser requirements fulfilled by effluent irrigation is difficult as it varies from project to project. To assess the requirements accurately, it is recommended to collaborate with an agronomist during project design, based on expected effluent nutrient concentrations.
Exercise:
Let’s assume we would like to estimate the required area, for the effluent of 100m3/d to irrigate the Banana trees by the subsurface irrigation method.
The following data is provided.
- kc value for Banana Trees: 1.05
- Overall efficiency of water use (ETa) = 0.60 (subsurface irrigation)
- Potential evapotranspiration (ET0) = 200 mm/month
- Available wastewater production = 100 m³/day
Calculation:
To calculate the area required, we need to rearrange the equation to Ac = area of the crop
