Water Balance Methods

Irrigation scheduling tools that predict when to irrigate based on weather and crop conditions are prevalent, and many of these are available in the form of computer programs or websites. Current weather data is used to calculate an amount of water that would be evaporated by a reference crop such as grass, and then a crop coefficient is used to scale that reference value to a specific crop need. The majority of these programs are based on the water balance method. Scheduling irrigation using a water balance, or checkbook method, is based on the available water in the soil profile. Like a checkbook, inputs are credited to the total soil water, and withdrawals are debited from the soil water. The inputs to the soil water are rainfall and irrigation. Withdrawals include transpiration through the plant, evaporation from the soil surface, and deep percolation into lower soil layers. During the growing season, evaporation and transpiration, commonly termed “evapotranspiration” and abbreviated “ET” are the most important processes by which water is removed from the soil. The key to using a checkbook method is knowledge of the available water capacity of soils. In the Southeast Coastal Plain, our soils are typically more sandy and can only hold about 0.08 inches of water per inch of soil. Some scheduling tools combine soil moisture monitoring with water balancing. This approach can prevent over-irrigating early in the crop season or under-watering later during peak crop water use.