Determining the Water Retention Curve of Soil by Hanging Water ColumnJustin Huxel, Central Arizona College

Eduardo Bautista, Water Management and Conservation, USDA-ARS

Results

Conclusion

Abstract

Materials & Methods

Introduction

Discussion

Irrigation management requires an understanding of how water flows in the soil. The flow of water is a function mainly of gravity and the matric potential (tension) exerted by the soil. The matric potential depends on soil texture, structure, and water content. This project aims to measure the relationship between water content and matric potential (tension) for a group of soil samples. This data will be used to develop a water retention curve, which could eventually be used for modeling water flow in the soil. Currently, ALARC scientists measure this relationship using pressure plates. However, those devices are useful only for measuring tension vs. water content at tensions greater than 1/3 bar. For irrigation purposes, it is more important how water content behaves at tensions less than 1/3 bar. Therefore, it was necessary to develop and test Hanging Water Columns in order to measure water content at lower tensions.

The Hanging Water Column works due to the principles of communicating vessels. If the vessels are under atmospheric pressure, water levels in communicating vessels are the same due to hydrostatic equilibrium. If we lower one vessel (a burette in our setup), water will flow out until achieving a new equilibrium. A porous plate in one vessel (a funnel) prevents air, and therefore, water from flowing. This creates sub-atmospheric pressure conditions below the porous plate. The pressure differential causes suction which takes the water out of a soil sample placed on top of the porous plate.

The soil used for this project was sieved of particles larger than 2mm and dried The sample is then compacted into circular ring to achieve a bulk density of about 1.55g/cm^3, which should be representative of bulk density in the field.

A Buchner funnel with a porous ceramic plate is connected with a 3-way stopcock and a burette by flexible PVC tubing. The system is then completely filled with tap water and then placed on ring stands. Water should be added or drained so that the height of water in the burette is equal to the height of the top of the porous plate.

The soil, ring and a piece of circular filter paper are placed on top of the porous plate in the funnel with the filter paper in contact with the plate and soil. Add water into the system via the burette until the water level has equalized to the top of the soil. The burette is then lowered by a set increment and the water level in the burette is recorded. It usually takes approx. 10 minutes to ensure the system has reached equilibrium for a 5cm decrease in height. The maximum tension allowable is dependent on the pressure rating of the porous plate. Pressure above this rating will result in air entering the system and further results will not be accurate. This tension was about 60cm for the course-pored plate and above 140cm for the fine-pored plate.

After the system has reached the desired tension or has failed by allowing air to enter the system, the “wet” weight of the soil is recorded. The soil is then dried in an oven at 105°C for 24 hours. The weight of the oven-dried soil is the “dry weight”. The volumetric water content, θ, can now be calculated for the any tension value. Θ = (wet- (water level for selected tension – water level of final tension reading) - dry weight)/dry weight * bulk density

The general shape of the measured Water Retention Curve (Figure 2.) is consistent with theory. For comparison purposes, Figure 3 shows a theoretical curve calculated with the Saxton water retention model for soils of the same texture as our samples. In both cases, as the sand content increases, the water content decreases for a given tension. The main difference from the model s the water content at saturation, which is too high for our experiments. This is still true even when compared to typical saturation values reported for most soils. The saturated water contents at the lowest tension are about 10% higher than the Saxton model. Even when looking at soils with an extreme amount of clay, they only rarely have a water content above 50%. This indicates a substantial source of error which has not been resolved at this time. One possible reason for this error is that the current procedure for both the hanging water column and the pressure plates use disturbed, loose soil that has been sieved of particles larger than 2mm. The dry soil sample is compacted in the laboratory to achieve a bulk density representative of the bulk density in the field. This alters the soil’s structure and means that the soil sample is missing larger particles such as gravel and roots. This could have an extremely significant effect on the water retention curve and could be resolved by using undisturbed soil cores for future testing.

Soils samples from field F113 were analyzed for texture (percentage of sand, silt and clay) by the hydrometer method. The soils were then sorted by their percentage of sand. From this data, we determined the average sand percentage for field F113 was about 64%. The Hanging Water Column was used for tensions from 0cm to 140cm and pressure plate data was used for 1/3bar (336cm), 5bar (5100cm) and 15bar (15300). In all three graphs, the soil loses water at a significant rate at low pressures but the rate of water loss decreases at very high pressures, only losing about 1-2% of the volumetric water content between the last two readings of 5 bars and 15 bars.

In conclusion, the Hanging Water Column apparatus developed in this project can be a great tool in soil analysis, especially with conditions immediately following or during irrigation or precipitation when the soil is very wet. The results are consistent and the trends reflective of other models and previous data. However, it is crucial to resolve the issue of the high water content at saturation before utilizing this method and using the data to model water flow in soil.

Special thanks to Dr. Eduardo Bautista, Dr. Clinton Williams, Dr. Jarai Mon, Alan Knopf and the organizers of the Project Puente Internship Program.

Acknowledgements

ReferencesSoil Water Characteristics Program V 6.02Saxton, K. E., & Rawls, W. J. (2006). Soil water characteristic estimates by texture and organic matter for hydrologic solutions. Soil Science Society of America Journal, 70(5), 1569-1578.Dane, J.H. and Hopmans, J.W. 2002. Water Retention and Storage. In: Methods of Soil Analysis. Part 4. Physical Methods. Soil Sc. Soc. Am. Madison, WI

Figure 2. Water Retention Curve Determined by Experiment

Figure 1. Methods of determining Water Retention Curve for Different Tensions

Figure 3. Saxton Model Predicting Water Retention Curve

Secondly, using a laboratory lattice instead ring stands would provide more accurate results and allow for a more permanent set up. The ring stand only allows for about 60cm of working space before cinderblocks are needed to allow for additional height. Because of this, the tubing may not actually be hanging but resting on the surface. This can result in variations in the water level due to the pressure on the tubing by being partially supported. In addition, excess water could be present either in or on top of the porous plate that could contribute to elevated water level changes. This could be resolved by ensuring there is no loose water on top of the plate before beginning the experiment or be a simple calibration of the data to account for water not from the soil Finally, there are additional variables that were not accounted for and could have an effect on the water retention curve, such as organic matter and salinity. However, it is very unlikely that these factors contributed significantly, to the high water content.