Crop covers reduce soil erosion and surface runoff by protecting the soil surface from physical raindrop impact and by increasing water infiltration rates. Cover crops are being promoted as sustainable management practices in some Hawaiian agriculture watersheds. Little is known about the impact of these cover crops on some soil physical properties, i.e. infiltration, soil hydraulic conductivity and soil water retention. The main goal of this work was to study the impact of three cover crops (sunn hemp, sudex and oats) on the surface and subsurface water movement. The field work was conducted on an Ewa silty clay loam soil, Waialua, Oahu. Tension Infiltrometer (TI) was used to measure surface infiltration under a variable pressure ranging between -41 and 0 cm. The Guelph Permeameter (GP) was used to measure the saturated hydraulic conductivity at 15 and 30 cm depths. Saturated and unsaturated hydraulic conductivities were measured for the three cover crops and the control, fallow. Soil hydraulic conductivities measured with TI increased with an increase in the pressure of the water application from -41 cm to 0 cm. Results of the GP showed that the fallow treatment had the lowest saturated hydraulic conductivity measured at both depths. However, saturated hydraulic conductivity at the 15 cm depth was higher than at the 30 cm for three out of the four treatments. Results of this study are needed for a companion water flow and solute transport simulation work.

Cover crops are being promoted as sustainable management practices in some Hawaiian agriculture watersheds. The Hawaiian Islands have many areas of agricultural lands that exhibit elevation gradients and are very susceptible to soil erosion and surface runoff. Several studies have proved that the presence of cover crops will reduce erosion and runoff. These cover crops may protect the soil surface from physical raindrop impact and increase water infiltration rates. However, only little attention has been given in Hawaii regarding the effects of cover crops in reducing soil erosion and surface runoff. In the study of water and contaminant transport, several parameters are needed in order to understand the physical characteristic of water and solute in soil profile. The key parameters are soil hydraulic conductivity, water content and water retention. In accordance with Fares at al. (2000), the relationships of those parameters are needed for crop growth, management of irrigation and drainage, and the modeling of water flow and chemical transport through the soil. Moreover, Unsaturated Flow Apparatus (UFA) Ventures (2004) website states that “proposed regulations from the United States Protection Agency will soon require measurement of unsaturated transport parameters for each geologic unit, soil horizon and engineer component for modeling and performance assessment needs”.

Hamdhani*, A. Fares, and M.H. RyderCollege of Tropical Agriculture and Human Resources (CTARH), University of Hawaii

Research Site

This field study was conducted in an area within the property of Pioneer HI-Bred International, Inc at 67-172 Farrington Hwy Waialua, located on the north shore of Oahu. Table 1 shows selected soil physical properties for a typical Ewa silty clay loam soil (Hawaii Soil Survey Bulletin). These are laboratory measured values which might be different from those measured in the field. The field measurements were undertaken during the period between March and April 2004.

1. Hydraulic conductivity measured with the GP showed more variability across treatments than hydraulic conductivity measured with TI.

2. Saturated hydraulic conductivity values were higher than unsaturated hydraulic conductivity values.

3. Saturated hydraulic conductivity values seem to be influenced by the method with which the measurements were made.

4. Saturated hydraulic conductivity measured with GP showed more variability than saturated hydraulic conductivity measured with TI.

Appreciation is extended to Pioneer Hi-Bred International, Waialua Oahu.Also thanks to Drs. C. Ray and M. Sanda at Civil & Environmental Engineering Department.

Benson, Craig. H., and Gribb, Molly. M. (1997). Measuring unsaturated hydraulic conductivity in the laboratory and field. Proceedings of sessions on unsaturated soils at geo-logan ’97.

Clothier, B., Scotter, D. 2002. Unsaturated water transmission parameters obtained from infiltration. Methods of Soil Analysis. Part 4- Physical Methods.

Fares, A, Alva, A.K., Nkedi-Kizza, P, and Elrashidi, M.A. 2000. Estimation of soil hydraulic properties of a sandy soil using capacitance probes and Guelph permeameter. Soil Science.

Unsaturated Flow Apparatus (UFA) Ventures. (2004, April 23) Hydraulic conductivity. Retrieved from http://www.ufaventures.com/ufa_ventures/tech_briefs/hydraulic _con.html

Contact address (corresponding author):CTAHR-NREM Sherman Building 101, East-West Road, Honolulu, Hawaii, USA.Tel: (808)956- Email: hamdhani@hawaii.edu

Field Procedures

Guelph PermeameterThe two instruments used to measure the saturated and unsaturated hydraulic conductivities were the Guelph Permeameter and the Tension Infiltrometer. The Guelph Permeameter is an in-hole, constant head permeameter which employs the Mariotte principle. The measurements were taken in four different plots (treatments) at a depth of 15 cm and 30 cm. After boring a hole, the GP was placed, and then its reservoir was filled with water. The water was slowly flowing into the auger hole and then penetrating into the soil. The readings of the water level falling in 2 to 5-minute time intervals were recorded until a constant falling rate was reached. These steps were followed at each measurement level.

The objective of this study was to evaluate the impact of different land use cover crops (sunn hemp, sudex and oats) and fallow on saturated and unsaturated hydraulic conductivities as measured by:

• Guelph permeameter and•Tension infiltrometer

Ewa Soil Properties

Depth Clay Moist Bulk Permeability Available Water

Density Capacity

(cm) (<2mm) (G/cm3) (cm/hour) (cm/cm)

0 - 45.54 35 - 50 1.1 - 1.3 1.5 - 5.1 0.1 - 0.12

0 - 45.54 35 - 50 1.1 - 1.3 1.5 - 5.1 0.09 - 0.11

45.54 - 151.8 35 - 40 1.1 - 1.3 1.5 - 5.1 0.1 - 0.12

Source: Hawaii Soil Survey Bulletin No: HI31-9-1

Research site

Guelph Permeameter

Tension InfiltrometerThe Tension Infiltrometer allows measurement of infiltration with a constant negative pressure head at the soil surface. A porous plate covered with a cloth on the base of the device allows suction to be constantly maintained inside during the measurements of the unsaturated hydraulic conductivity. The reservoir tube of the TI is filled with water, a thin layer of sand is spread on the measurement area to ensure a good contact between the soil surface and the porous plate. Readings of the water level falling were taken periodically, every 5 to 7 minutes using two suction levels 42 and 0 cm.

Tension Infiltrometer Hydraulic conductivity measured by Guelph Permeameter

Treatments Ksat (cm/hour)

  Depth of 15 cm Depth of 30 cm

Oats 5.3 16.45

Sudex 5.81 1.81

Sunn hemp 18.24 1.7

Fallow 1.7 0.85

Ks measured by GP in Oats Plot

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Ks measured by GP in Sunn hemp Plot

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Ks measured by GP in Fallow Plot

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Ks measured by GP in Sudex plot

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Hydraulic conductivity measured by Tension Infiltrometer

Treatments Hydraulic Conductivity (cm/hour)

  Tension 41.6 cm Tension 0 cm

Oats 0.5364 1.0728

Sudex 0.4032 4.8204

Sunn hemp 0.6696 2.142

Fallow 2.6784 4.2876

With the exception of the Oats treatment, Ksat decreased with depth, thus, Ksat values at 15 cm depth are 2 to 10 orders of magnitude higher than those at 30 cm. Regardless of the soil depth, Ksat values for the fallow treatment were the lowest as compared to the other treatments.

K measured by TI at Oats plot

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K measured by TI at Sudex plot

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K measured by TI at Fallow plot

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•Saturated hydraulic conductivities were higher than unsaturated hydraulic conductivities across all the treatments.

•Oats and Sudex treatments had the lowest and highest saturated hydraulic conductivities among all the treatments, respectively.

•Fallow and Sudex treatments had the highest and lowest unsaturated hydraulic conductivities among all the treatments, respectively.

•With the exception of Fallow treatment, the unsaturated hydraulic conductivity values of the other treatments were very close.

K measured by TI at Sunn hemp plot

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H33A-67