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Why do you sometimes see fields being irrigated (watered) while it is raining? Isn’t this a waste of valuable water?

This question is definitely a valid concern that is often shared with us. There are a variety of factors that come into play when thinking about irrigation. The short answer is that farmers might irrigate while it is raining because only 12% of crop water needs are met by rain. Luckily, both rain and irrigation water are stored in the soil and can be used days later.

Let’s think about the amount of water a crop needs so that it can produce a successful yield. Out of the theoretical total amount of water a crop needs on average throughout the watershed (100%), 59% comes from irrigation and only 12% of the crop’s needs are actually met by rain during the growing season. The remainder comes from a combination of soil moisture obtained from snowmelt during the winter and reduced crop yield when water is not sufficient to meet the theoretical needs.

Here’s some things to consider:

  • A very small percentage of the irrigation needs of a crop are met by rainfall.

  • Irrigation is done over the entire season with the entire crop yield in mind. Farmers account for precipitation when determining the full season water needs of the crop.

  • From a logistical perspective, changes made to the outflow of water in the system take a long time to make (often days or longer). By the time the change is made, the rain would have already long stopped, and the crops would still need to be watered.

  • Some areas may have soil with good drainage or poor drainage, meaning that even after a rainstorm the soil remains dry at various soil depths where plant roots need water. In this case, irrigation is often required to ensure that the plants receive enough water to grow.

  • Different crops have varying water requirements, and their optimal growth conditions will not align with the rainstorm patterns. Certain crops might need specific amounts of water at different stages of their growth cycle.

The good news:

Any water used on a crop while it is raining is not a waste of water. Both rain and irrigation water are stored in the soil and available to plants days later. If the rain is higher than expected on a given day, less irrigation is needed the following week.

If irrigation occurs when it is raining, is it taking more water away from fish than needs to be taken?

The Drought Planning Management Committee has streamflow targets in place to ensure that there is enough water available in the river for fish habitat during the irrigation season, no matter how much is being diverted (taken out) for irrigation, and these targets were created in partnership with the Henry’s Fork Foundation.

How it works:

The only reaches of the Henry’s Fork where fish populations are impacted by water supply are:

1. Trout and Kokanee populations upstream of IPR – dependent on the amount of water that stays in the reservoir all summer

2. Trout populations between IP Dam and Riverside – limited by available winter habitat, which depends on outflow from the dam

3. Trout populations downstream of St. Anthony – habitat limited by mid-summer low streamflow and high water temperatures

What HFF is doing to provide more and cooler water for fish:

1. Keep more water in Island Park Reservoir via water conservation

2. Develop managed aquifer recharge strategies that can increase inflow of cool groundwater to streams and reservoirs

3. Increase precision in water management via daily data sharing and collaboration



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