INVESTIGATION OF WATER MOVEMENT DURING TURF IRRIGATION
Harrison B. Rhodes
Dr. Eng. Sci, CIH (Retired)
CSU Advanced Master Gardener
ABSTRACT
This investigation was undertaken to learn more about the details of the water movement in the soil during the irrigation of turf. Of particular interest was the movement of "gravity water" and it's relationship to the field capacity and wilting point during the irrigation process.
The experimental program consisted of a period of days where the turf was irrigated with about 0.9 inches of water every third day. The irrigation program used was selected in accordance with the CSU Master Gardener recommendations, i.e. irrigate bluegrass turf to a depth of about 9 inches measured by screwdriver penetration and water again when the drying had reduced the penetration to 2 - 3 inches.
The test site was a basically level, 8 foot by 8 foot area, in a well established (about 20 year old) low traffic, bluegrass lawn with no discernible thatch layer. The area was in full sun from about 9:30 AM to 3:30 PM and again at 5:00PM. Rain gauges were placed at each corner of the 8 foot square and at the corners of an interior square and were used to check the amount of water put down in each test.
The water movement was monitered by measuring the daily changes in the soil moisture profile. This was done by the collection of a combination of corer and auger soil samples each morning and evening. The collected samples were segmented, weighed wet, oven dried and reweighed to provide the weight percent water as a function of depth. This technique provides a direct measure of the water content profile in the soil each day.
For the irrigation procedures and soil profile present in these tests, the data shown support the following statements:
- At the initial condition after water is applied, most of the water is in the top 3 inches where the water content ranges from field capacity to saturation (essentially filling the voids).
- From about 3 to 8 inches, the water content decreases steadily with depth from field capacity to a moisture content of about 1 to 3 weight percent above the wilting point at about 8 inches down.
- The concept that, when turf is irrigated, "gravity water" drains out rapidly i.e. in a day or so, leaving the soil wetted at field capacity for a substantial portion of the wetted depth, does not occur. Most of the water applied remains in the top 3 inches of soil until it is removed by drying or used by the turf.
TABLE OF CONTENTS
1. INTRODUCTION (Click on Graphics for Full View)
This investigation was undertaken to learn more about the details of the water movement in the soil during the irrigation of turf. Of particular interest was the movement of "gravity water" and it's relationship to the field capacity and wilting point during the irrigation process.
The experimental program consisted of a period of days where the turf was irrigated with about 0.9 inches of water every third day. The water movement was monitered by measuring the daily changes in the soil moisture profile. This was done by the collection of a combination of corer and auger soil samples each morning and evening. The collected samples were segmented, weighed wet, oven dried and reweighed to provide the weight percent water as a function of depth. This technique provides a direct measure of the water content profile in the soil each day.
2. SELECTION OF THE IRRIGATION PROGRAM
The irrigation program used was selected in accordance with the CSU Master Gardener recommendations, i.e. irrigate bluegrass turf to a depth of about 9 inches measured by screwdriver penetration and water again when the drying had reduced the penetration to 2-3 inches. This pattern was attained using Rainbird impact sprinklers operating for 15 minutes at 1,3 and 5 AM. The split applications were used to avoid or minimize puddling. Total water application was about 0.9 inches.
The test site was a basically level, 8 foot by 8 foot area, in a well established (about 20 year old) low traffic, bluegrass lawn with no discernible thatch layer. The area was in full sun from about 9:30 AM to 3:30 PM and again at 5:00PM. Rain gauges were placed at each corner of the 8 foot square and at the corners of an interior square and were used to check the amount of water put down in each test.
4. SAMPLE COLLECTION AND PROCESSING
Soil samples were collected at about 9 AM in the days irrigation ran and at 8 AM on other days. Evening sample collection started at about 7 PM. A distance of 8 to 12 inches was left between successive holes.The procedure for the corer samples was as follows:
- Screwdriver penetration was first measured. These data were needed in order to compare soil water levels with penetration depth. This also made it possible to avoid large roots or rocks that would interfere with corer penetration.
- The corer was then inserted to its limit of penetration with rotation and reasonable force.
- The corer was next removed and the contents divided into segments, usually about 1 inch long, and placed into small, pre-tared aluminum baking tins. Tare weights ranged from 4.24 to 4.35 grams.
- Each sample was weighed immediately on an Ohaus beam balance graduated to 0.1 gram.
- After weighing, each sample was oven dried at about 225 degrees fahrenheit. The initial corer samples were dried for at least 2 hours.
The remainder of the depth was sampled using an earth auger. The auger was fitted with six-inch section of heavy wall plastic pipe that fit snugly but smoothly over the bit portion of the auger. Collection procedures were as follows:
- The auger was driven about 2 - 3 inches into the soil at the bottom of the hole. It was then slowly and carefully withdrawn from the hole with the bit portion sliding into the the plastic pipe as it emerged from the hole. This procedure minimized the amount of the frequently quite dry sample that was lost or fell back down into the hole.
- When using the auger there was some material near the tip that was loosened but was left behind when the auger was pulled up. This material, plus any that had fallen back down the hole while the auger was pulled up, was removed carefully using a 1/2 inch O.D. copper tube. The tube was pressed downward on the loose material at the bottom of the hole. The material contained enough water to compact inside the tube and remain there for removal. It usually took two passes with the auger and copper tube to remove the desired 3 - 5 inches of sample.
- These samples were weighed and placed in the oven to dry in the same manner as those collected with the corer. Since this was a slower procedure for each sample, only one or two samples were collected at a time and were promptly weighed to avoid moisture losses to the air.
- Finally, all of the dried samples were removed from the oven and weighed promptly.
5. FIELD CAPACITY, WILTING POINT AND AVAILABLE CAPACITY
These properties are based on soil texture. Combined samples for the various depths were analysed by the CSU Laboratory at Fort Collins. The results, together with the corresponding soil properties from Reference (1), are listed below:
| Weight Percent |
Weight % Water |
||||||
| Depth | Sand | Silt | Clay | Texture | FC | WP | AC |
| 0-4" | 48 | 34 | 18 | Loam | 23.6 | 9.2 | 14.4 |
| 4-8" | 50 | 30 | 20 | Loam | 23.6 | 9.2 | 14.4 |
| 8-12" | 57 | 26 | 17 | Sandy Loam | 15.4 | 5.8 | 9.6 |
6. PRESENTATION AND DISCUSSION OF RESULTS
The movement of water in the soil will be examined here for two cases:
- Excellent drying conditions for the entire three days with ET values ranging from 0.26 to 0.32.
- Poor drying conditions with ET values of 0.06 to 0.24.
The daily drying patterns for the test sequences are presented in Figures 1 and 3. In each figure the weight percent water is plotted on the horizontal axis as a function of depth on the vertical scale. Although the samples were collected as discrete segments of one to four inches, each result is plotted as a single point at each respective midpoint depth.
For each test sequence, the curves shown on the figures are for the cores taken on the morning and evening of the day the water was applied, the evening of the second day and the evening of the second or third day. Each curve can be readily identified by the symbol used together with the type of line. The values for the field capacity and the wilting point described previously for each soil zone are shown by vertical dashed lines.
6.2.1 Test sequence #1 (Figure 1)
The initial morning conditions about three hours after the end of the third increment of water addition is shown by the open circles and the solid line. Note the region down to about 3 inches where the water content substantially exceeds the field capacity. There is a region below three inches where the water content decreases steadily below the field capacity until it reaches, at a depth of eight inches, a value about 2 weigh percent greater than the wilting point.
Day 1 was hot, dry and sunny with an ET value of 0.30 inches. The only drying that occurred was a small amount in the top inch. (Compare the circles to the squares.) The water content in this inch was about 39 weight percent or 49 percent by volume. The pores (~ 50% voids) at the start of the day were essentially saturated with water with little or no room for air movement. This may have impeded the normal drying during this first day.
More important, however, is that at the end of this first day, approximately 15 hours after the last application of water, the water content vs. depth profile has not changed significantly from it's initial shape. There is no indication that "gravity water" has moved downward from the upper 3 inches where the content is well above field capacity, into the lower depths where it remains well below field capacity.
Day 2 (Compare squares to triangles) was also hot and sunny with an ET value of 0.26 inches. A considerable amount of drying occurred down to 8 inches but there was still a region near the surface that was wetter than field capacity.
Day 3 (Compare triangles to solid circles) had an even higher ET of 0.32 inches and there was substantial drying over the entire depth to 9 inches. There were no longer any water contents greater than field capacity.
6.2.2 Test sequence #2 (Figures
2 & 3) 
This sequence represents a case where drying conditions were poor, i.e. cloudy with ET values of 0.06 and 0.24 inches. The first examination of the depth of the water distribution was recorded at 9 AM, three hours after the end of the last application of water. Figure 2 shows the change in water content at each increment of depth. The solid areas represent increases. The blank areas show very small regions of apparent drying which are too small to be significant. It is immediately evident that, at this time, the great majority of the water applied is in the top 3 inches of soil.
The changes that occured during drying, starting with the wet condition just
described, are given in Figure 3, expressed in the same manner as in Figure
1. During the first day (compare circles to squares), with an ET of only 0.06
inches, very little drying occurred. There is evidence of a moderate amount
of wetting from about 3 1/2 to 9 inches. This movement, however, occurs in
a region where the water content is already well below field capacity. The
mechanism would appear to be capillarity, not gravity flow.
During the second day (compare squares to triangles), the ET value was higher at 0.26 inches and fairly substantial drying occur at all depths. This included the region below 3 1/2 inches where the wetting had taken place during the previous day. The key point, however, is that two days after water was applied there is still a substantial amount of water at levels well above the field capacity in the top 2 inches of soil. There is, again, no evidence of movement of water by gravity flow.
For the irrigation procedures and soil profile present in these tests, the data shown in Figures 1 and 3, support the following statements:
- At the initial condition after water is applied, most of the water is in the top 3 inches where the water content ranges from field capacity to saturation (essentially filling the voids).
- From about 3 to 8 inches, the water content decreases steadily with depth from field capacity to a moisture content of about 1 to 3 weight percent above the wilting point at about 8 inches of depth.
- The concept that, when turf is irrigated, "gravity water" drains out rapidly i.e. in a day or so, leaving the soil wetted at field capacity for a substantial portion of the wetted depth, does not occur. Most of the water applied remains in the top 3 inches of soil until it is removed by drying or used by the turf.
USDA, "Scheduling Irrigations". Edited by Harold R. Duke. U.S. Government Printing Office: 1991 - 834 - 690
Placed on the Internet February 5, 2000
Updated May 31, 2009