To start with, there seem to be two different camps on the issue of fall irrigation, one from the perspective of irrigators and water managers and the other from that of people wanting to keep the water in the river to maintain flow levels. In writing this article, I prefer to come from the perspective which I know best: plant, soil, and water quality issues. Are there any documented benefits to fall irrigating? Sure, there are a lot of good reasons for irrigating in the fall after the heat of the summer season is over. But, should irrigators be fall irrigating? Irrigators need to make sure they know why they are fall irrigating, the consequences, and what they hope to accomplish. Basically, whether an irrigator should be fall irrigating or not depends on the purpose.

Reasons for Fall Irrigating
1.   Fall irrigation encourages germination of volunteer grain and fall weeds, allowing better control with herbicides and tillage. By fall irrigating, a crop producer can establish a relatively uniform flush of weeds and volunteer grain under vigorous, stress free growth conditions. All of those conditions improve the efficacy of weed control by herbicides. If the objective of tillage is to control fall weeds and volunteer grain, then fall irrigation will enhance that operation as well.

2.   Fall irrigation facilitates fall tillage, whether for overwinter soil conditioning or for organic matter decomposition. Tillage in the fall is performed for a variety of reasons, including distribution of the work load, timeliness to facilitate spring planting operations, and residue management, either to remove the residue or facilitate its decomposition. Regardless of the reason, raising the soil moisture level prior to tillage facilitates the tillage operation while at the same time reducing power requirements. Soil strength is inversely proportional to the soil water content. What that means is the drier the soil, the stronger the soil, and vice versa. By irrigating just to the point of wetting the upper soil depth to field capacity, a producer can facilitate the tillage operation and reduce the time and energy required to complete the operation. Another important aspect here is the decomposition of organic matter. Typically at the end of the cropping season, the surface soil is very dry, possibly even approaching the wilting point. At this water content, organic matter decomposes quite slowly . Microbes need moisture; therefore, added moisture, up to a point, facilitates organic matter decomposition.

3.   Fall irrigation supplies water for fall plant growth in pasture, grass, and alfalfa crops. Alot of pasture and hay ground in Montana is predominated by cool season grasses, plants which put on most of their growth during the cool months of spring and fall. This is when they photosynthesize, storing sugars and carbohydrates in the root system. Most folks can quickly recognize the change of seasons from spring to summer to fall by the way the grass in their lawn grows. This also holds true for hay and pasture crops. Begin irrigating the first of September when the days are getting shorter and the nights cooler and you'll see good grass growth. This grass growth is a potentially valuable source of winter feed

4.  One thing that irrigators need to be aware of when it comes to fall irrigating for plant water use is that alfalfa, as tough as it is, is pretty fragile when it comes to playing with fall water. Generally, a vigorously growing alfalfa crop needs about six weeks of "conditioning" before freeze up when growth begins to slow and water use drops off. Top growth might be killed back by frost several times, but root growth continues into October and even November. Added moisture from fall irrigation facilitates root growth and repeated top growth prior to conditioning, but irrigators should not irrigate alfalfa beyond late September. This will allow for that six week conditioning or hardening period. Also, because of their deep root systems, alfalfa can be damaged by frost heaving in ground that froze with a high moisture content. This doesn't apply to grasses, which are much hardier and able to continue growing right up to freezing with no conditioning period. Because grasses are predominantly supported by broad, diffuse rooting systems, they are not subject to the damages of frost heaving.

5.  Fall irrigation leaches salts. Now, there's a reason not many irrigators think about. Under alot of irrigation conditions in Montana, because of the limited amount of water available during the irrigation season and the greater evaporative demand, the net movement of water during the irrigation season is upward, meaning soluble salts from the irrigation water and from the soil are drawn upward toward the soil surface. Off season irrigation can move those salts down below the root zone. Off season irrigation water can also fill the soil pores, enhancing the dissolution and diffusion of those salts from the fine pores so that when moisture comes from rainfall or snowmelt, the salts are leached further down, below the root zone. Actually, in most cases in Montana, irrigators unknowingly rely on this over-winter leaching of salts to satisfy the necessary leaching requirement.

6.  Fall irrigation increases stored soil moisture. This one seems to be up for debate. Some folks say it doesn't do any good, because it all evaporates before spring. Well, that's not true! There have been some studies which have shown on a very limited, random sampling that soil moisture that comes in the fall evaporates by the time planting season rolls around. The logical argument against this is then, ‘What is the sense of fallowing?' An extensive amount of research by scientists in both Montana and North Dakota has shown that essentially all the moisture stored in the soil, excluding that from irrigation, comes between the months of September and December and again between March and June. About 40% of the recharge moisture comes in the four fall months.

So, now the question becomes ‘Can you outguess nature?' If you fall irrigate, what are the chances of getting more moisture during those high precipitation months than the soil can hold? In other words, are you wasting water by fall irrigating? Your guess is as good as mine, but you can play the odds. On a sandy soil, the available water storage capacity is about 1.5 inches per foot. So, if you have 4 feet of dry soil, let's say at wilting point, then you can store 6 inches of water. Anything greater than that is lost from the soil, but it does contribute to ground water and stream base flow during the dry months. In this scenario, if you fall irrigated and added 3 inches of water to a sandy soil, you'd still be able to store another 3 inches of rainfall and snowmelt.

Compare that to a silt loam soil which stores 2.2 inches of available water per foot off soil. That means it can store between 8 and 10 inches of water, if it's dry to wilting point. What's the likelihood of getting 8 inches of effective rainfall between September and December? One must also consider the fact that 50% or more of our snow cover sublimates, meaning it goes directly from snow on the surface to a vapor, without having a chance to infiltrate the soil. These are pretty good arguments for adding some of that moisture in the fall.

Fall irrigation has added rainfall effects. The amount of research that has been conducted on the benefits of "added rainfall effects" is mind boggling. Back in the days when we were seriously looking at making it rain, and more recently as we have looked at the effects of global warming on climatic patterns, one of the issues that has been exhaustively studied is that of added rainfall. Does it really make a difference if we get 1 additional inch of moisture during the non-growing season, and does it matter whether it comes from rainfall or irrigation? You bet it does! Even researchers in Montana have shown that a single inch of additional moisture can have a significant effect on grass and forage production. And, MSU researchers have beat to death the notion that an additional inch of moisture is worth anywhere from 4 to 8 bushels of wheat per acre and significantly more than that for oats. An additional inch of moisture translates into anywhere from 400 to 500 pounds of alfalfa per acre.

7.  Now there are a few vague areas that need attention, namely ground water recharge and contributions to sustained baseflow. Not surprisingly, this year we heard a lot of complaints from homeowners in the northwestern part of the Gallatin Valley about their wells going dry during August. It seemed strange to them, but when you think about it, it made perfectly good sense. Over the years, as residential development has taken over the prime agricultural land of the Gallatin Valley, alot of land which was under very inefficient flood irrigation and serviced by very inefficient gravel and earth-lined irrigation ditches and canals has been taken out of production. With the "sloppy" irrigation gone, alot of water that used to go to ground water recharge now goes to watering lawns, and few homeowners water their lawns to the point of having significant leaching losses. In addition, land that used to be covered by crops is now covered by roof tops, city streets, parking lots, and sidewalks. Most of the rainfall that lands on those roof tops and asphalt doesn't go into ground water recharge. Instead, it goes into storm drains and down the river.

What all this leads to is the notion that some fall irrigation might be good for shallow ground water recharge and for a slow pipeline of water to stream baseflow. During the winter months, stream baseflow depends on ground water seepage into the streams.

Well, the long and the short of it is that there are some legitimate reasons for taking a serious look at fall irrigation. The more important question is ‘What do you hope to accomplish by fall irrigating?' Clearly we are not saving moisture, since eventually it all falls into the same hydrologic cycle and cycles through the oceans and back to the atmosphere. But, by fall irrigating, we might be able to create a little "added value" for a resource that most of us take for granted, which tourists pay good money to get in a bottle but pay better money to fish in, which often begins its journey from the atmosphere in the hydrologic cycle someplace here in Montana, and which always seems to be in short supply here at the top of the watershed.

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