Almond Irrigation Scheduling: Sourcing ETo
When determining plant water demand, the current environmental conditions must be taken into consideration. These factors, which include temperature, solar radiation, wind speed, humidity, and percentage of soil cover, influence both the rate of evaporation from the soil and the rate of transpiration from the plant (also known as evapotranspiration). By knowing these variables, we can calculate the evapotranspiration of the plant of interest.
Research over the past 50+ years has refined the ways we estimate evapotranspiration. Initial estimates used to be performed using evaporation pans (Epan). Epan filled with water would be manually monitored for water evaporation on a daily basis. Water evaporation was then correlated with water use by fully irrigated grass (ETo) through the use of lysimeters. Fast forward a few years, with the development of more precise electronics, weather stations demonstrated the ability provide accurate measurements of the required variables. Using these data, a correlation between measured weather variables and ETo was developed. This equation, known as the Penman-Monteith equation, or some variation, is now used by nearly every weather station to estimate ETo.
Having the ability to identify the rate of ETo is the foundation of irrigation scheduling. Although it is variable due to day-to-day environmental fluctuations, it is accessible through multiple sources. Within California, State funded weather stations placed throughout California measures the required water variables. These values are reported on the CIMIS web-site and are available free of charge. Within other almond growing areas, similar programs also exist, but are not as thorough. Thankfully, many weather stations can provide an estimation of ETo – as long as they have the ability to measure the required variables. Furthermore, ETo information is often made available in many publications, including trade newspapers, websites, and extension offices. Regardless of where the ETo information is sourced, it is important to be consistent with its use to account for any possible inherent errors.
Identifying a resource that provides an accurate estimation of this number is the first step to irrigation scheduling. Once ETo is known, it needs to be multiplied by the crop coefficient (Kc) to determine a specific plant’s water use. After this calculation, the evapotranspiration of the crop of interest (ETc) is known. Water applications meeting this value should be applied to prevent water stress. Since the application is usually by an irrigation system, the total amount of water applied must account for the inefficiencies of the system (Ea). The age of the plant and percentage of canopy coverage should also be taken into account when working with perennial crops. This is especially true with younger, smaller plants which have less canopy coverage and therefore use less water. Lastly, the total amount of water applied should not exceed the soil’s water holding capacity or infiltration rates. If so, water losses may occur, reducing the efficiency of the irrigation.
Included are some photos of resources that could be used to estimate ETo. Figure 1 is a screenshot from the CIMIS weather station within California. This resource provides the daily ETo (second column) reported as inches per day. Other weather variables, including temperature and rainfall, are also provided for reference. It is a free resource for California farmers, but weather stations may not always be close to the specific farm. Experience suggests, however, that the ETo values are still “within the ballpark” and are accurate enough to use. The summation of ETo values between any two irrigation events is the total ETo that should be used when determining how much to irrigate. These values would then be multiplied by the Kc and other factors to determine total water needs.
The second figure is from a weather forecasting service. This figure is projecting ETo water use (mm/day) based on anticipated weather conditions. These amounts can be used to determine how many hours should be scheduled to irrigate for the coming week. Since it is a projection, however, the hours may need mid-week revision if weather conditions change or rainfall occurs.