Improving irrigation scheduling is among the best Climate-Change adaptation options. Increasing agricultural water-use efficiency might reduce the impact of reduced hydraulic resources and climate variability.
According to the last IPCC report Climate Change means an important challenge to agriculture. Particularly, irrigated agriculture might face a significant reduction in water availability.
Furthermore, the temperature rising would increase the crop water needs. Therefore, farmers need improving irrigation scheduling and increasing water-use efficiency.
Improving irrigation scheduling comprises two basic questions: How much to irrigate? and When to irrigate?
Improving irrigation scheduling means that water applications are chosen according to soil properties.
”Field Capacity” (FC) is the amount of water, expressed as soil moisture, retained in the soil after drainage.
It is usually estimated as soil-water content at a pressure of -0.33 bar.
The FC depends on soil mechanical composition, as well as on soil structure and organic matter content.
Irrigation water supply must bring soil moisture only up to FC. The reaming water application will be lost by soil infiltration.
On the other hand, “Wilting Point” (WP) is the minimal soil water content the plant requires not to wilt. It can be estimated as soil-water pressure at -0.15 bar.
The “Available Water Content” (AWC) is therefore the difference between FC and WP. However, WP is actually undesirable. Instead, the concept of “Readily Available Water” (RAW) has been introduced. It is usually a percent of AWC, from 80% to 50%, according to the crop importance. RAW means that the minimum water content allowed in the soil before irrigation is higher than WP.
The irrigation water table Ir (How much to irrigate?) can be estimated from:
Ir (mm) = Cr . RAW / Ei
where Cr is the root depth and Ei is the irrigation efficiency.
Crop consumes soil water through Evapotranspiration (ETc), sum of leaves transpiration and soil evaporation.
The ETc can be calculated from meteorological data. This is usually a free service to farmers, from meteorological institutions in many parts of the world.
When to irrigate?. Just when soil water content approaches to the lower limit of the RAW.
We must therefore know the current soil water content at a daily basis. Many irrigation systems use moisture sensors, which trigger irrigation sprinklers just in the moment when soil moisture reaches the RAW lower limit and closes them when the moisture reaches the FC. This is a brilliant solution, but expensive.
Instead, a simpler choice is to estimate soil water contents through water balance. This method considers the main water inputs and outputs.
Neglecting runoff and infiltration, soil water content in day d (SWCd) is the sum of soil wáter content in the previos d-1 day (SWCd-1), plus rainfall (R), plus irrigation (I) less evapotranspiration (ETc):
SWCd = SWCd-1 + R + I – ETc
Following this simple daily balance the farmer can estimate when soil water content approaches the lower RAW limit and therefore when to irrigate.
