Right now the Lovelock Valley is looking at one of the lowest water allocations for irrigation in years. So, what are farmers supposed to do if the water allotment is not increased? 

I wish there was an answer that fit every operation, because every farm is different and with less than 12 inches of water per acre to irrigate with, that does not leave a lot of options for producers.

First, let look at some Alfalfa Irrigation Facts:

1. A general rule of thumb is that it takes 6 inches of water to produce one ton of alfalfa per acre.

2. It takes approximately 0.5 tons per acre for it to be economical to harvest the field (depending on the prices of hay).

3. In a 4-cut system, the first cutting usually makes up about 35-38% of the year’s total forage yield.

4. Alfalfa’s evapotranspiration (ET) rates averages varies from 0.1 to 0.2 inches of water/day from April 1 to May 1, 0.2 to 0.3 inches/day from May 1 to Sept. 30, and from 0.1 to 0.2 inches/day from Oct. 1 to November 1. Extremes in ET may be about 0.4 inches/day in mid-summer.

5. Under moderate moisture stress, alfalfa plants have the ability to go into a drought-induced dormancy. If the plant has adequate carbohydrate reserves, the plant should survive until moisture is returned.

6. Deficit irrigation may change the quality of the hay produced. Water stress reduces stem growth relative to leaf growth and the leaf to stem ratio may be higher with deficit irrigation. Therefore, hay quality may actually be increased with deficit irrigation.

Where do we go from here? First, you need to understand the term “Evapotranspiration” (ET). Evapotranspiration (ET) is the standard term for quantifying crop water use and is measured in inches of water over the planted area. ET accounts for water that enters the atmosphere through plant leaves (transpiration) as well as water that goes directly from the soil into the air (evaporation). ET rates are dependent on climatic factors such as solar radiation, air temperature, humidity and wind speed. Warmer the temperatures and lower the humidity, increase ET rates. Therefore, the plant uses water more efficiently in the spring and fall when temperatures are lower and the humidity is higher.

The bottom line, though, is that drought conditions mean lower alfalfa yields. The key to making the best of the situation is to identify strategies that will maximize the profitability of an alfalfa operation during periods of restricted water supply. Blaine Hanson, Steve Orloff and Dan Putnam from the University of California Cooperative Extension provide the following three strategies for coping with drought.

Strategy 1: Reduce the irrigated acreage.

Strategy 2: Start early in the crop season with full irrigation and continue with full irrigation until the water supply is used up. No irrigations will occur for the rest of the crop season. 

Strategy 3: Practice deficit irrigation over the entire crop season by applying smaller amounts of water than the crop would require.

Strategy 1 (reduced acreage) - This drought strategy is to reduce the number of irrigated acres to match the available water supply. 

The reduced acreage is fully irrigated to obtain maximum ET and maximum yield per acre, following normal irrigation practices. 

The remaining acreage is not irrigated and so has no yield, but the first-cutting yield for the entire acreage may still be good if sufficient moisture remains in the soil from winter and spring rainfall and snowmelt. 

One concern with this strategy is that the grower needs to make the reduced water supply last the entire crop season.

When you consider that there is a potential for further water supply reductions later in the year, you can see that the grower’s ability to implement this strategy may be trumped by any additional cuts toward the end of the season.

Strategy 2 (full irrigation followed by no irrigation) - This drought strategy is to start irrigations early in the crop season and fully irrigate the entire acreage until the water supply is used up. After that, there can be no more irrigation for the remainder of the crop season. 

The number of cuttings a grower can get using this method depends on how much water is allocated. The acreage will have maximum ET and maximum yield per acre during the fully irrigated period, but little or no ET or yield during no-irrigation period that follows. This strategy maintains high yields for early harvests and foregoes any expectation of yields later in the season. One advantage of this strategy is that the grower uses the entire water allocation earlier in the crop season, before any additional reductions can be implemented.

Strategy 3 (deficit irrigation) - This strategy is deficit irrigation of the field throughout the crop season, either by means of applying less water per irrigation, reducing the number of irrigations per cutting, or implementing some combination thereof. This strategy reduces ET as well as yields between harvests and is not recommended if the grower only has access to very small amounts of water, since the yields that are possible from such small water applications may not be economical to harvest.



Which Strategy Is Best?

The best strategy is the one that provides the largest returns to land and management in your particular situation and that depends on two factors: how much revenue the crop can generate under reduced yields and how costly it is to implement the particular strategy. 

All of the major variable production costs are associated with either irrigation or harvest. Variable production costs per acre per harvest are the same for strategy 1 (reduced acreage) as for a fully irrigated field, but because strategy 1 irrigates only part of the production acres, the farm-wide production costs are lower. 

For strategy 2 (full irrigation followed by no irrigation), variable production costs per acre per harvest are the same as for a fully irrigated field during the fully irrigated period, but the grower incurs no variable costs during the no-irrigation period. 

Irrigation and harvests costs for strategy 3 (deficit irrigation) should be lower than those for a fully irrigated field, but higher compared to strategy 1 (reduced acreage). 

However, along with these variable costs there are fixed costs that do not change with particular strategies. Fertilizer and pest control costs may be the same for all three strategies since these costs generally occur early in the crop season, before irrigation becomes necessary.

In summary, the key to deficit irrigation strategies for alfalfa, begins with understanding that the crop yields best in the spring and early summer, because of lower ET rates and higher Irrigation Water Use Efficiency (IWUE is defined as the amount of hay produced per unit of irrigation water applied and can be expressed as tons of hay per foot of water). 

In a 4-cut system, the first cutting usually makes up about 35-38% of the year’s total forage yield.

The key is not to look at the annual growth cycle as a whole, but to look at individual cuttings. 

Therefore, utilizing Strategy 2 (full irrigation followed by no irrigation), may optimize your profitability by taking advantage of lower ET rates, higher water efficiency (IWUE) and reduced variable costs. 

The highest yielding and highest quality production is in the first several cuttings of the year. Late cuttings of alfalfa are typically 50-60-% the yields of those first cuttings.

 Therefore, in a drought year, it’s important to water early, to fill the profile, maximize production during the early period. 

If one has to stop irrigating in mid-summer due to lack of water, the early growth will make up for the lack of production later.

For more information on this topic go to: https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=46142