**University of California**

**UC Drought Management**

# ET I. S. Under Sprinkler, Furrow, and Border Flood Irrigation

**Step 1**: Determine the soil moisture stored in the crop's root zone

**Step 2**: Determine the Allowable Depletion Amount - the amount of soil moisture depletion at which irrigation should begin.

**Step 3**: Track crop ET to determine when cumulative crop ET since the last irrigation is equal to the Allowable Depletion.

**Step 4**: Determine the irrigation amount.

## Step 1: Stored Soil Moisture

The stored soil moisture is the soil moisture the plant can draw on to meet its evapotranspiration (ET) demands. It is the product of the soil's waterholding capacity and the crop rootting depth.

Stored Soil Moisture (in) | = | Soil Waterholding Capacity (in/ft) | x | Rooting Depth() |

** Example:** Almonds with a 5 foot root depth. Silt loam soil

Assume the silt loam soil had 1.5 in/ft of waterholding capacity

Stored Soil Moisture (in) | = | 1.5(in/ft) | x | 5 ft | = | 7.5 inches |

## Step 2: Allowable Depletion Amount

To minimize the water stress to the plant, irrigation should occur prior to all the stored soil moisture being depleted. The recommended level (%) of stored soil moisture depletion to minimize water stress is referred to as the Allowable Depletion. In general, the allowable depletion for high-value crops such as vegetables should be less. When information is not available, a 50% or less allowable depletion can be assumed.

**Allowable Depletion Amount**

The allowable depletion amount is the product of the stored soil moisture and the percentage allowable depletion.

Allowable Depletion Amount (in) | = | Stored Soil Moisture (in) | x | Allowable Depletion (%) |

**Example:** For almonds under sprinkler, furrow or flood irrigation, assume the allowable depletion is 50%.

Allowable Depletion Amount (in) |
= | 7.5 (in) | x | 50 (%) | = | 3.75 in. |

The Allowable Depletion Amount is often referred to as the Net Irrigation Amount.

Step 3:

Track crop ET to determine when cumulative crop ET since the last irrigation is equal to the Allowable Depletion Amount.

Use either real-time ET or historical ET information to track crop ET.

**Example:** Use historical ET information for almonds in Modesto during June 1-15. From table, almond ET (in/day) would be:

**3.28 in. / 15 days = .22 in/day**

The irrigation interval is the allowable depletion amount divided by the crop ET:

Irrigation Interval (days) | = | Allowable Depletion Amount (in) | / | Daily Crop ET (in/day) |

Irrigation Interval (days) |
= | (3.75 in) | / | (.22 in/day) | = | 17 days |

## Step 4: Determine the irrigation amount.

No irrigation system is 100% efficient so additional water will need to be applied. The irrigation amount, accounting for inefficiencies, is the Gross Irrigation Amount.

Gross Irrigation Amount (in) |
= |
Net Irrigation Amount (in) Irrigation Efficiency (%) |
x | 100 |

Typical irrigation Efficiencies for sprinkler and surface irrigation systems are listed below:

Irrigation System |
Irrigation Efficiency (%) |

Basin flood | 70-80 |

Border strip | 70-80 |

Furrow | 65-75 |

Sprinkler | 75-85 |

**Example:** The Net Irrigation Amount (Allowable Depletion Amt.) from above is 3.75 inches. Almonds are irrigated with border strip irrigation (assume an irrigation efficiency of 75%)

## Summary of example:

During the beginning of June, irrigation should occur approximately each 17 days and 5 inches of water should be applied.