10.2: Advance, Recession, and Infiltration

To the casual observer, surface irrigation looks like a very simple concept. The water is applied at the inlet end and the irrigator allows gravity to move the water across the field. As the water moves across the field, part of it infiltrates and part of it is stored on the soil surface. After the water reaches the end of the field, runoff occurs unless the flow is blocked by an earthen dike. Water is usually not applied to the entire field simultaneously but rather is applied to only a portion of the field at one time. These portions of the field are referred to as sets. A set may be an individual border strip, a single basin, or a group of furrows. The water is applied for a fixed time period called set time.

Even though the concept of surface irrigation appears simple, the science of surface irrigation can be very complicated. This is largely because of the many interactions that occur between the rate of inflow, land slope, roughness of the land slope, uniformity of the land slope, and most importantly, the infiltration rate of the soil during irrigation.

In surface irrigation the soil infiltration rate has a large impact on the ultimate distribution of water and the ultimate amount of water that runs off the edge of the field. This is in contrast to sprinkler and microirrigation where the hardware of the system has more control on how the water is distributed and whether or not the water infiltrates at the desired location. The hardware can be designed so that the application rate is less than the infiltration capacity of the soil allowing all of the water to infiltrate at the point of application. This is not true with surface irrigation. Once the water leaves the inlet end of the field, the manager no longer has control of the water; the soil now has control. Infiltration during surface irrigation can vary significantly on land that is cultivated annually. It depends upon whether it is the first irrigation of the season or whether it is a subsequent irrigation, and where tractor tires have traveled and compacted the soil. Some of the variations in infiltration are illustrated in Figure 10.6. There can be many other sources of infiltration variability within the field.

In practice, many surface irrigators have developed an art of irrigating, rather than applying science to irrigation management. What we hope to do in this chapter is to balance the two: the art and the science. It is unlikely that we will ever get to the point where we can completely manage based on theory alone because there are so many variables that are out of the manager’s control.

Figure 10.6. Trends in cumulative infiltration as influenced by irrigation sequence and wheel traffic.

Figure 10.7. Advance and recession curves for surface irrigation.

Let us take a look at the fundamentals that apply to surface irrigation. The first concept is advance and recession of water. In Figure 10.7, two curves are shown: the advance curve and the recession curve. The advance curve is a graphical picture of how rapidly water moves from the inlet end to the downstream end of the field, which can be measured directly in the field (Figure 10.8). The curve is not linear. As water moves further and further from the inlet end, the rate at which the wetting front moves decreases. It is typical that it takes about one-third as much time to get halfway across the field as it does to get from the starting point to the downstream end of the field. For example, if it took 3 hours to get to the midpoint of the field, we would estimate approximately 9 hours total to reach the downstream end.

Figure 10.8. Students measuring stream size, advance, recession, and runoff in a furrow irrigation system. (Photo courtesy of Laszlo Hayde, IHE Delft Institute for Water Education.)

The recession curve is a plot of how the furrow drains after irrigation has been stopped, and can also be measured directly. Usually, the surface begins to drain from the upstream end. For the example illustrated in Figure 10.7, drainage occurs in approximately 1 hour. This is in contrast to the advance time, which was 9 hours before water reached the downstream end.

Why are advance and recession important? The amount of water that infiltrates at any point in the field depends upon how long water was at that point. In our example in Figure 10.7, water was at the inlet end for 12 hours because irrigation was continued for 3 hours after water reached the downstream end of the field. At the downstream end, water arrived after 9 hours of application. Further, the recession took approximately 1 hour at the downstream end. That is, recession stopped at hour 13. So, how long was water present at the downstream end? In this case, 4 hours (13 - 9). At the upstream end water infiltrated for 12 hours, while at the downstream end, water had the opportunity to infiltrate for only 4 hours. You can now see why the amount of infiltrated water would not be uniformly distributed.

The time difference between the recession curve and advance curve is called opportunity time. The opportunity time curve shown in Figure 10.9 is the time difference between the advance and recession curves in Figure 10.7. In this example, opportunity time decreased as you move from the inlet end to the downstream end of the field. If the infiltration characteristics of the soil are uniform throughout the field, we would expect more infiltration at the inlet end compared to the downstream end.

Figure 10.9. Opportunity time for surface irrigation.

What is necessary to achieve good uniformity? For perfect uniformity the opportunity time curve would have to be horizontal, i.e., equal at all locations within the field. This can only happen if the advance curve and recession curve are parallel to one another. In other words, advance time would have to equal recession time at all points in the field. Even though we commonly picture more opportunity time at the inlet end than at the downstream end, it sometimes happens that recession is slower than advance. In this case, opportunity time would increase with distance from the inlet end.

Now, let us look at the development of the infiltration distribution profile. In Figure 10.10, we illustrate an example relationship between the cumulative infiltration and opportunity time. The data are listed in graphical as well as tabular form.

Figure 10.10. Example infiltration vs. opportunity time.

In Table 10.1, the advance time, the recession time, and the opportunity time have been tabulated. By combining the opportunity time information with the infiltration characteristics of the soil you can determine the infiltration at any position. Use 600 feet as an example distance. Here the advance time was 2.7 hours and recession occurred at 12.5 hours. Thus, the opportunity time was 9.8 hours. From Figure 10.10 we find that the infiltration would be approximately 3.6 inches. A similar procedure can be followed to obtain infiltration at any point along the furrow. The infiltration distribution curve shown in Figure 10.11 is based on the data from Table 10.1. Since the opportunity time decreased with distance from the inlet end, the infiltration also decreased with distance. We will return to this example later as we develop the relationships between water applied, infiltration, runoff, and the effective amount of water stored in the soil.

Figure 10.11 Infiltration profile.

Table 10.1. Data for Figures 10.7, 10.9 and 10.10

Distance (ft)	Advance Time (h)	Recession Time (h)	Opportunity Time (h)	Infiltration (in)
0	0.0	12.0	12.0	4.2
300	0.8	12.2	11.4	4.0
600	2.7	12.5	9.8	3.6
900	5.5	12.8	7.3	3.2
1200	9.0	13.0	4.0	2.4