Irrigation

The SPAW model performs a one dimensional, single field, water budget of the specified climate-crop-soil situation. The size of the field may be as small as a point or as large as conditions remain sufficiently uniform. Non-uniformity's such as the different depths of water which occur longitudinally in furrow irrigation can be considered only by including such differences as input conditions in separate runs. The method of irrigation largely only affects the water budget by the vertical flow of water in the control volume. Only with sprinkler irrigation does interception of water by foliage occur. Many other important irrigation processes such as the hydraulic behavior of water in furrow irrigation or the impact of droplet size on the soil under sprinkler irrigation are not considered. Runoff from an irrigated field is not usually described by the SCS curve number procedure; therefore the default value for run-off from irrigation water is zero, however runoff from rainfall is estimated.

The irrigation simulation options provided in the model permits much flexibility in water application. Effective use of water in irrigation requires careful attention to scheduling, that is, when to irrigate and how much water to apply. To help irrigation planners with scheduling decisions, the irrigation options include ten methods to determine when to irrigate and six to determine how much water to apply. Some options permit entry of known or otherwise specified irrigation data. Others specify conditions to be met before irrigation is accomplished, thus causing the irrigation schedule to be estimated by the simulation. Options selected must be used throughout the growing season, and cannot be changed during the season.

The time and depth options included in the model are summarized below. Operational detail on the options are presented in the Screen Help. Some combinations of the time and depth options may be unreasonable, thus careful selection and definition are required.

Time Options for irrigation:

  1. Fixed interval: Irrigation occurs on an established initial date and at the user-specified interval in days thereafter.
  2. Fixed dates: Irrigation occurs on user-specified dates.
  3. Soil water depletion of the root zone: Irrigation occurs when the soil moisture within the root zone is depleted above user-specified limit.
  4. Soil water depletion of total soil profile: Irrigation occurs when the soil moisture within the total soil profile is depleted above user-specified limit.
  5. Soil water tension at a specified depth: Irrigation occurs when calculated soil moisture tension in a selected layer exceeds a user-specified value.
  6. Soil water tension in root zone: Irrigation occurs when calculated soil moisture tension weighted by root distribution exceeds a user-specified value.
  7. Accumulated PET: Irrigation occurs when the calculated accumulation of potential evapotranspiration since the previous irrigation exceeds a user-specified value.
  8. Accumulated AET: Irrigation occurs when the calculated accumulation of actual evapotranspiration since the previous irrigation exceeds a user-specified value.
  9. Vegetative stress index (moving average): Irrigation occurs when the moving average of the daily vegetative stress index exceeds a user-specified value.
  10. Yield reduction index (moving average): Irrigation occurs when the moving average of the daily yield reduction index exceeds a user-specified value.

Depth Options for Irrigation:

  1. Fixed depth for all irrigations: A fixed depth of water applied at each irrigation.
  2. Specified depths for each irrigation: A user-specified depth of water is applied for each irrigation.
  3. Refill the root zone: The water added will increase the simulated soil moisture level in the root zone to a user-defined upper limit, which is typically field capacity but which may be greater or less than field capacity.
  4. Refill the total soil profile: The water added will increase the simulated soil moisture level in the total profile to a user-defined upper limit, which is typically field capacity but which may be greater or less than field capacity.
  5. Accumulated PET: The depth of water applied is a user-specified percent of the potential evapotranspiration accumulated since the previous irrigation.
  6. Accumulated AET: The depth of water applied is a user-specified percent of the actual evapotranspiration accumulated since the previous irrigation.

Field Hydrologic Processes | Precipitation | Infiltration | Potential ET
Interception | Soil Water Evaporation | Plant Transpiration | Crop Water Stress
Root Water Uptake | ActualET | Soil Water Redistribution