Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
I was recently asked about irrigating snap beans prior to harvest. Irrigation is a critical management tool for producing high yielding and high-quality vegetable crops. Scheduling irrigation for different vegetables grown under center pivot, travelling gun, or solid set overhead systems involves knowledge of the soil water holding capacity, the effective rooting depth of the crop (how deep water can be drawn by the crop), how efficiently water is being delivered (water losses to evaporation before it reaches the crop and how much water is lost to runoff), how much water is being used by the crop (transpiration) and how much water is being lost from the soil and wetted surfaces directly (evaporation). The combination of transpiration and evaporation losses is termed evapotranspiration.
To schedule irrigation, the goal is to replace water lost through evapotranspiration without excessive runoff or excessive loss through percolation out of the root zone. Another factor to consider is the permissible water depletion; how much will you allow the soil to dry down between irrigations. For most crops we set this at 50% of the water holding capacity of the soil. However, for some shallow rooted crops you may want to keep that value lower (only allow for 30% depletion between irrigations). By knowing how much water is being lost and how much is left in the soil, you can determine when to irrigate and how much to irrigate.
In general, vegetable water use at the start of the rapid growth phase is 0.10 to 0.15 inches per day and increases weekly to full canopy where water use is from 0.25 to 0.33 inches per day. For example, with cucumbers, water use 20 days after planting was 0.13 inches per day, water use 40 days after planting was 0.27 inches per day, and water use at peak (50 days) was 0.30 inches per day.