Subsurface Drip Irrigation in Gravel-Filled Cavities - GeoScienceWorld

Subsurface Drip Irrigation in Gravel-Filled Cavities Alon Ben-Gal,* Naftali Lazorovitch, and Uri Shani

Reproduced from Vadose Zone Journal. Published by Soil Science Society of America. All copyrights reserved.

ABSTRACT

charge decreases. Shani et al. (1996) measured 6 to 8 m of back pressure in soil for a delivery system operating at 10-m line pressure. Such a decrease in the pressure gradient can result in considerable reduction of flow from noncompensating emitters. Variations in soil hydraulic properties within a field cause discrepancy in pressure buildup adjacent to individual drippers and subsequent nonuniform discharge and water application (Warrick and Shani, 1996). Additionally, when a positive head is generated in the saturated zone surrounding emitters, paths of least resistance to water flow and ensuing pressure alleviation can lead to surfacing (chimneying) and ponding of water, thus defeating the aims and purposes of SDI. Attempts to avoid surfacing include fastidious attention to design criteria where flow rates and depth of laterals take soil hydraulic properties into full consideration, so the saturated zone around the emitter remains under the surface and pressure buildup is minimized. In spite of such care in system design, local irrigation water surfacing is often observed in SDI fields, as illustrated in Fig. 1. Once surfacing begins, the vigor of the preferential flow pathway makes implementation of corrective measures difficult if not impossible. We hypothesize that a medium with relatively large pores replacing the otherwise saturated zone of soil in the immediate vicinity of drippers can prevent buildup of positive pressure and subsequent nonuniform application and surfacing phenomena. The specific objectives of this study were to define tubular cavities and columns of gravel whose volumes and dimensions are a function of irrigation rate, dripper spacing, and soil hydraulic properties and to evaluate such gravel-filled cavities against conventional SDI in a vineyard irrigated with municipal wastewater effluent. The method presented has commercial potential in irrigation where high-volume subsurface application of water is required. Gravel-filled cavities can be used in irrigation or waste disposal projects where surfacing of effluent or other low-quality water is absolutely prohibited. While there may be limitations to the feasibility of substituting soil with imported media, there is precedent for such methodology in other applications. Meshkat et al. (2000) replaced soil with coarse sand to reduce evaporation from surface drip irrigated fields. A number of commercial orchards in Israel use buried volcanic tuff in significant volumes in the root zone to enhance nutrient management efficiency (Tuff Merom Golan; see http:// www.tuff.co.il/news/engl_content.asp?contid⫽177).

Subsurface drip irrigation (SDI) is regularly used to provide water and nutrients to plants while maintaining a dry soil surface. Problems associated with the practice of SDI are spatially dependent reductions in dripper discharge and possible surfacing of water resulting from positive pressure at the emitter–soil interface. These can be resolved either through prudent care in matching dripper flow rates to soil hydraulic properties or by otherwise providing conditions under which positive pressure cannot arise. We present a method where water is applied to the soil within a gravel-filled cavity. The necessary volume of gravel is determined by the contact area between the cavity and the soil and is a function of irrigation rates, dripper spacing, and soil hydraulic properties. A theoretical solution for the radius of a gravel-filled cavity based on the perimeter of the saturated zone from a line source in the soil demonstrates that larger cavities are needed as soil hydraulic conductivity decreases. The method was tested using a numeric simulation model (HYDRUS-2D) and was used and tested in a vineyard of table grapes (Vitis Vinifera L. cv. Sugraone) in a 7-yr study with SDI and gravel-trenched subsurface application of effluent and fertilizers.

S

ubsurface drip irrigation is regularly used to provide water and nutrients to plants while maintaining a dry soil surface. Subsurface drip irrigation also contributes to the alleviation of health hazards, odor, contamination of groundwater, and runoff into surface water (Trooien et al., 2002). Drip emitters in SDI systems are positioned within the soil to conserve water, control weeds, minimize runoff and evaporation, increase longevity of laterals and emitters, ease use of heavy equipment in the field, and to prevent human contact with low-quality water (Camp, 1998; Lamm, 2002). Another motivation for using SDI is savings of the extensive labor involved with seasonal installation and collection of surface drip system laterals. Drippers are commonly buried 7 to 30 cm under the soil surface but are found as deep as 100 cm (e.g., in date palm [Phoenix dactylifera L.] orchards at Yotvata, Israel). Successful use of SDI depends on technological solutions to a number of obstacles. A subsurface drip emitter typically has a limited cavity around it into which water can freely flow. As the pore space at the dripper outlet fills with water, infiltration of applied water is limited by the hydraulic properties of the soil, and a positive pressure develops (Shani and Or, 1995; Shani et al., 1996). As pressure develops outside the emitters, dripper dis-

A. Ben-Gal, Department of Environmental Physics and Irrigation, Agricultural Research Organization, Gilat Research Center, M.P. Negev, 85280, Israel; U. Shani and N. Lazorovitch, The Hebrew University of Jerusalem, Department of Soil and Water Sciences, Faculty of Agriculture, Food and Environmental Quality Sciences, The Hebrew University of Jerusalem, P.O. Box 12 Rechovot 76100, Israel. Received 3 Mar. 2004. Original Research Paper. *Corresponding author ([email protected]).

THEORETICAL CONSIDERATIONS AND SOLUTION Philip (1992) presented an analysis of the conditions near a continuously flowing subsurface point source.

Published in Vadose Zone Journal 3:1407–1413 (2004). © Soil Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA

Abbreviations: GRAV, treatment with driplines placed in gravel trenches; SDI, subsurface drip irrigation.

1407

1408

VADOSE ZONE J., VOL. 3, NOVEMBER 2004

Table 1. Soil properties used for theoretical solution and numerical simulations (after Carsel and Parrish, 1988).† ␪r

␪s

␣VG

n

KS

I

␣G

0.5 0.5 0.5 0.5 0.5

m ⫺1 6.1 7.2 12.9 22.6 29.4


s⫺ 1

Clay Clay loam Loam Sandy loam Sand

0.068 0.095 0.078 0.065 0.045

0.38 0.41 0.43 0.41 0.43

0.8 1.9 3.6 7.5 14.5

1.09 1.31 1.56 1.89 2.68

m 5.56E-07 7.18E-07 2.89E-06 1.23E-05 8.25E-05

† ␪r, residual soil water content; ␪s, saturated soil water content; ␣VG, parameter in soil water retention function (van Genuchten); n, parameter in soil water retention function; KS, saturated hydraulic conductivity; I, pore connectivity factor; ␣G, exponent in unsaturated hydraulic conductivity function (Gardner).

Re ⫽ Fig. 1. Surfacing observed for a 4 L h⫺1 dripper in Hazerim Loess soil buried 30 cm below the surface.

Philip’s main conclusion was that a saturated region develops adjacent to the source, where water is under positive pressure. Philip assumed that in most soils a cavity is likely to be formed around the emitting point source. Warrick (1993) used an approach similar to that of Philip (1992) and described the saturated bulb under steady-state conditions from a quasilinear line source. Warrick presented a nearly cylindrical saturated zone with radius (R0): R0 ⫽

冤冢

冣

4 2␲KS ⫺␥ exp ⫺ ␣ ␣Q

冥

[1]

where KS is saturated hydraulic conductivity, ␣ is a soil constant (Gardner, 1958), Q is source discharge, and ␥ ⫽ 0.577216 is Euler’s constant. The required dimensions of a cavity around a dripper line that will prevent pressure buildup are a function of the equivalent surface area as determined by R0. Shani and Or (1995) similarly adapted Philip’s approximation to obtain an estimate of the equivalent radius of the cavity surrounding a single emitter (Re) as a function of any soil water pressure head at the source (P):

Fig. 2. Radius of a cylindrical area (R0), as a function of soil hydraulic parameters KS and ␣, satisfying conditions for no buildup of pressure in the soil. Solution of Eq. [1] for Q ⫽ 2.22 ⫻ 10⫺6 m3 s⫺1 m⫺1.

2Q␣ 8␲KS (␣P ⫹ 1) ⫹ ␣2Q

[2]

Shani et al. (1996) measured Q changes due to pressure buildup in the soil surrounding emitters. They found greater decreases in Q for soil hydraulic properties associated with fine-textured soils, greater decreases in Q as nominal emitter increased, and less change in Q when large pores were found near dripper outlets. We solved Eq. [1] for the case of drip irrigation in date palm orchards in the southern Arava Valley of Israel for Q ⫽ 2.22 ⫻ 10⫺6 m2 s⫺1 (8 L h⫺1 m⫺1). From a sensitivity analysis for R0 (Fig. 2), the range of KS and ␣ values for soils where backpressure from drippers would otherwise arise is demonstrated. The radius of the porous area needed to fulfill the requirement of no pressure buildup is a function of soil type and generally increases as soil texture becomes finer and KS and ␣ decrease. Combinations of KS ⬍ 1 ⫻ 10⫺6 m2 s⫺1 with ␣ values ⬍5 m⫺1 demand impractically large R0 values (⬎0.5 m). Alternatively, sandy soils with KS ⬎ 1 ⫻ 10⫺5 have negligible R0. Analysis of R0 from Eq. [1] as a function of Q for three soil textural size classes, whose properties are found in

Fig. 3. Radius of cylindrical area (R0) as a function of equivalent linear flow rate (Q ) satisfying conditions for no buildup of pressure in the soil for sand (KS ⫽ 8.25 ⫻ 10⫺5 m2 s⫺1; ␣ ⫽ 29.4 m⫺1), loam (KS ⫽ 2.89 ⫻ 10⫺6 m2 s⫺1; ␣ ⫽ 12.9 m⫺1), and clay (KS ⫽ 5.56 ⫻ 10⫺7 m2 s⫺1; ␣ ⫽ 6.1 m⫺1) soil types.


www.vadosezonejournal.org

Fig. 4. Effect of soil texture on water distributions from a gravelfilled cavity. Results from HYDRUS-2D after 10 h of simulated irrigation or at onset of surfacing. Water application rate of 2.22 ⫻ 10⫺6 m2 s⫺1 within a 5-cm-wide gravel-filled column. (A) Clay (surfacing at 45 min); (B) loam (surfacing at 8 h); (C) sandy loam; (D) sand.

Table 1, is shown in Fig. 3. The ␣ parameter of the soils, ␣(G) in Table 1, was estimated by equating the Kirchhoff potential (φ) (Gardner, 1958): φ⫽

0

冮⫺∞K(␺) d␺

[3]

of the Gardner and Mualem–van Genuchten (van Genuchten 1980) hydraulic models by substituting ␣G ⫽ KSφ ⫺ 1. In Eq. [3], ␺ is soil water potential. While R0 is negligible and not relevant for the sandy soil at flow rates of Q ⬍ 5.5 ⫻ 10⫺6 m2 s⫺1, increasingly larger porous areas are required as Q increases to avoid pressure buildup (Fig. 3). At the highest flow rates shown (2 ⫻ 10⫺5 m2 s⫺1) the sand soil results in R0 ⬎ 3 cm, the loam soil demands R0 up to 16 cm and the clay soil up to 36 cm. METHODS Numerical Simulation The HYDRUS-2D Version 2.0 code (Simunek et al., 1999) includes a two-dimensional finite element model for simulat-

1409

Fig. 5. Effect of column width on water distribution from a gravelfilled cavity. Results from HYDRUS-2D after 10 h (or until onset of surfacing) for Q ⫽ 2.22 ⫻ 10⫺6 m2 s⫺1 simulated water applications within gravel-filled column in a loam soil. Column widths of (A) 5 cm (surfacing at 8 h), (B) 10 cm, (C) 20 cm, and (D) 40 cm.

ing flow and transport in variably saturated media, was used to simulate axisymmetric water distribution patterns from a subsurface source. Simulations using HYDRUS-2D were made for water application in trench-shaped gravel-filled cavities to evaluate the effects of soil texture and gravel cavity dimension. Modifications of the HYDRUS-2D code allowing for ponding of water before infiltration and allowing for backpressure buildup around the cavity source were applied (Lazarovitch et al., 2005). The simulated soil profile was 1.5 m wide and 1 m deep with no-flow boundaries on the sides and free drainage of water from the bottom boundary. Initial Table 2. Initial properties of Arava Sandy Loam.† Sand, % Silt, % Clay, % ␪s ␪r Ks, m s⫺1 pH EC, dS m⫺1 Cl, mg kg⫺1

73.6 13.6 12.8 0.41 0.06 1.16 ⫻ 10⫺5 7.8 2.2 62

† ␪r, residual soil water content; ␪s, saturated soil water content; KS, saturated hydraulic conductivity; EC, electrical conductivity.


1410


Fig. 6. Gravimetrically measured water content in soil from samples taken midway between irrigation events from soil profile transects perpendicular to vine rows. Numerical data in figures show placement of sampling and represent average values from three replicate sections of the vineyard at Arava Research and Development Field Station. (A) Subsurface drip irrigation (SDI treatment); (B) gravel-filled trench irrigation (GRAV treatment). Circular markers indicate location of drip line.

conditions of the soil were near the permanent wilting point. Soil water flow properties applied were default values, from Carsel and Parrish (1988), given by the program for the various texture classes and are given in Table 1. Numerical simulations of water flow from the linear source in the gravel-filled cavities were based on maximum daily irrigation requirements of date palm plantations in the Southern Arava Valley of Israel (12 mm d⫺1 ⫽ 10 h at 8 L h⫺1 m⫺1 of drip line). Gravel in the simulations had a 10% porosity. The cavities in the simulations were gravel-filled columns extending from the soil surface to a depth of 40 cm. Drippers were situated near the bottom of the columns. Simulations were executed for each of four soil textural size classes (sand, sandy loam, loam, and clay) for a 5-cm-wide gravel cavity. For the loam soil, simulations were made for cavities of 5-, 10-, 20-, and 40-cm widths. Simulations allowed monitoring of water in the cavities and the surrounding soil through time. Successful combinations of soil, flow rate, and cavity size were those where the trenches did not completely fill with water during the irrigation event. Resulting water flow rates and distributions in the soil indicated that, as the soil hydraulic conductivity decreased, a greater cavity volume was necessary to guarantee that no pressure buildup of water in the soil occurred (Fig. 4 and 5). A 5-cm-wide, 40-cm-deep trench was insufficient for the assumed irrigation rate in clay and loam soils, but was success-

ful in preventing surfacing in the sandy loam and sand soils (Fig. 4). In the clay soil, surfacing occurred after 45 min of irrigation (Fig. 4A), and in the loam soil surfacing occurred after 8 h (Fig. 4B). Widening the trench in the loam soil to 10 cm successfully prevented surfacing of free water (Fig. 4B). As the trench size was increased to widths of 20 and 40 cm, not only was free water kept further from the soil surface, but the extent of nonsaturated moisture at the surface was reduced as well (Fig. 5C and 5D).

Vineyard Trial A 0.3-ha vineyard consisting of 400 Vitis vinifera L. cv. Sugraone grapevines on Salt Creek (Ramsey) (Vitis champini Planch.) rootstock planted in 10 rows with 3.5 by 2.0 m spacing was established in November 1995 at the research station of Arava Research and Development at Yotvata in the Arava Valley in Israel (29⬚53⬘ N, 53⬚3⬘ E). The soil in the vineyard was Arava Sandy Loam (Typic Torrifluvent). Initial soil properties are given in Table 2. The vines were irrigated for one season using Netafim (Netafim, Israel) Ram 17-mm 2.3 L h⫺1 in-line drippers spaced every 0.4 m and placed adjacent to rows on the soil surface. The irrigation system included automatic line flushing valves, air entry ports at the irrigation head, trifluralin (Treflan, Dow AgroSciences, Indianapolis, IN, EPA Reg. no.



1411

Fig. 7. Chloride content of soil samples taken midway between irrigation events from soil profile transects perpendicular to vine rows. Numerical data in figures show placement of sampling and represent averaged value from three replicate sections of the vineyard at Arava Research and Development Field Station (A) Subsurface drip irrigation (SDI treatment); (B) gravel-filled trench irrigation (GRAV treatment). Circular markers indicate location of drip line.

67219-250) coated filter disks for anti-root intrusion, fertilizer injection equipment, and computerized automation (EldarGal, Israel). Before winter pruning of the second season, the vineyard was divided into independently irrigated sections containing 21 to 24 vines each in three to four adjacent rows. Three such sections were used for each treatment where the drip lines were placed in one of two configurations: “conventional” subsurface drip buried at 40 cm in the soil (SDI), or subsurface trenches 40 cm deep and 20 cm wide in which the drip lines were placed and covered with gravel (GRAV). The gravel, with porosity of 40%, consisted of 5- to 15-mm-diameter particles with rounded edges. Water and fertilizer applications, plant protection, and other cultural practices were conducted according to local commercial methods and following recommendations by the local extension service. Periodical sampling and analysis of the municipal wastewater effluent used for irrigation revealed average values of the electrical conductivity of 2.4 to 2.8 dS m⫺1, pH of 7.4 to 7.8, and Cl contents of 520 to 620 mg L⫺1. Vines were trellised on four-wire Y-shaped systems. Pruning was conducted in December each year as recommended by the local extension service and as practiced in local commercial vineyards, retaining two long canes of 8 to 10 buds and four renewal spurs of two to three buds on each side of the trellises for each vine. During the next 6 yr, flow rates of the drip

systems were monitored, and plant growth as biomass from winter pruning and harvested grape yields was measured. In the spring of 2003, flow rates per section were analyzed and soil was sampled in transects between the rows in each section to investigate soil water content, distribution of Cl ions in the soil profile and root density distribution. Soil profiles were exposed in 1-m-deep pits extending 2 m from the vine rows. Soil samples were collected immediately after the profiles were exposed by first removing 20 cm of soil from the profile face and then collecting 10 by 10 by 10 cm cubes from 40 points within the profile. Analysis of Cl ions was accomplished by chloridometer (Chloride Analyzer 926, Corning, Medfield, MA). Water content and Cl concentration were measured for each sample, averages according to location within the soil profile were made for the three replicates of each treatment, and graphical isoline mapping was accomplished using inverse distance (power ⫽ 2) gridding using Surfer 6.01 (Golden Software, Golden, CO). Root distribution and density were evaluated qualitatively by washing a 1-cm thickness of soil and exposing roots on the profile wall. Water distributions at the mid-point of the irrigation cycle were measured in the spring after 6 yr of irrigation. The water content was highest around and underneath the drip line for each of the treatments (Fig. 6). Water was distributed radially around the SDI source with relatively high water content


1412


together showed greater spatial variation of flow in the SDI treatment than in the GRAV treatment. Total water allocation for the entire experiment in GRAV had ⬍1% difference among the three sections while there was 6% less water allocated to one of the sections of SDI compared with the other two. No significant differences were found between the treatments in regards to grapevine pruning biomass or for harvested fruit. Total pruning biomass for the 1997 through 2002 seasons was 48.2 kg (SD ⫽ 8.5) for SDI and 44.2 kg (SD ⫽ 15.5) for GRAV. Average annual harvested grape biomass for the same period was 16.1 kg per vine (SD ⫽ 4.0) for SDI and 14.5 kg per vine (SD ⫽ 2.9) for GRAV.

ANALYSIS AND CONCLUSIONS

Fig. 8. Roots exposed on soil profiles perpendicular to vine row in vineyard after 7 yr of treatments. (A) Subsurface drip irrigation (SDI treatment); (B) gravel-filled trench irrigation (GRAV treatment).

above as well as below the dripper. Water from the GRAV source had greater downward movement, with a concentric distribution pattern under the source. A dry soil surface was maintained for both the SDI and GRAV treatments. Chloride ion concentrations were found to be inversely correlated with water content, and Cl⫺ accumulated in the outer zones of the wetting patterns. Chloride distributions are shown graphically in Fig. 7, where relatively low concentrations of Cl are seen below and surrounding the water source and high concentrations as distance from the source increases. Accumulation of these ions at or near the dry soil surface was apparent for both the SDI and GRAV treatments. Root density distributions closely resembled wetting patterns. Exposed roots exhibited in photographs of the soil profiles from each treatment (Fig. 8) show greater horizontal distribution of roots for GRAV than for SDI. Average emitter flow rate neither changed with time nor was different between the different treatments, and no differences were found in the rates of flow from excavated drippers. Flow rates were slightly higher than the manufacturer’s specified rate of 2.3 L h⫺1. Average output per emitter calculated from total flow rate of each treatment before the end of the experiment was 2.39 L h⫺1 for the SDI treatment and 2.44 L h⫺1 for GRAV. Flow measured from 12 individual drippers excavated at the end of the experiment showed a high level of uniformity, with averages of 2.49 L h⫺1 (SD ⫽ 0.15) and 2.44 L h⫺1 (SD ⫽ 0.13) for SDI and GRAV, respectively. In spite of this, individual sections within a treatment irrigated

Theoretical consideration of buried cavities, numerical simulation of flow from gravel-filled trenched cavities and long-term observations of the method in a vineyard under conditions of high water demand together demonstrate the potential for water application in cavities for preventing backpressure at subsurface emitters and for eliminating surfacing. Cavity dimensions in such systems are a function of soil hydraulic properties and irrigation flow rate. Backpressure is expected to occur for commercial flow rates in many soils where individual emitters are in direct contact with the soil. The proposed method unifies the discrete emitters into a continuous cylindrical source and substantially increases the surface area between the source and the soil. Because of the significance of this transition, R0, and consequently, cavity size, necessary for most soil–flow rate combinations are relatively small. Comparison of conventional SDI with GRAV application methods in the vineyard showed more water and roots surrounding the SDI treatment and more spreading horizontally below the GRAV treatment. Dripper placement in gravel-filled cavities caused greater horizontal distribution of water, roots, and Cl compared with conventional subsurface drip. No reduction in biomass production or fruit yield was found between SDI and GRAV treatments for grapevines grown under the treatments for 6 yr. Greater field-scale variability in water distribution was found for SDI than for GRAV in the vineyard, indicating soil conditional differences in emitter flow rates. Commercial use of irrigation in gravel-filled cavities depends on technological methods of gravel and drip line installation. Gravel was added manually during preparation of trenches at the time of drip line installation in the vineyard field study. In a 30-ha commercial date palm plantation at Kibbutz Yotvata in Israel, 1-m-deep, 40-cm-wide trenches are dug by backhoe, a 10- to 15-cm layer of gravel is then distributed using a front-loader, a drip line is placed on the gravel surface, and the remainder of the trench refilled with soil. Subsurface drip installation methods where the dripper line is directly trenched into the soil could plausibly incorporate application of gravel as well. Uneven emitter outflow and surfacing are not expected to be problems in well-designed SDI systems. Optimization involves matching of flow rates and emitter spacing to the soil hydraulic conductivity. While backpressure,



and possible surfacing, is an issue for all buried sources, the use of pressure-compensated emitters can effectively regulate discharge rates. In spite of this, a number of situations warrant consideration of the GRAV method where the gravel, or other media providing large pores in the designated space, can permit higher flow rates, fewer drippers, fewer laterals, shallower emitter placement, cheaper (non-pressure compensated) emitters, and easier handling of problematic or nonuniform conditions. The proposed method can be utilized as a safety factor for particularly low-quality water where surfacing is absolutely prohibited. The method can be especially practical for problematic or nonuniform soils where design requirements for the “worst” combinations of soil and flow rates are impractical. Dimensions of the gravel-filled cavity can be manipulated to optimize management options. For example, by increasing cavity width, one can reduce the depth of emitters needed to ensure that the saturated zone does not reach the soil surface. The gravel-filled cavities can also be useful where high irrigation rates are required, such as in date palm irrigation and wastewater disposal projects. In all of these cases, the gravel allows successful subsurface application with substantially less investment in equipment. Assessment regarding suitability of the method is a matter of calculating cost of the gravel-filled cavities compared with cost of the equipment otherwise necessary for successful subsurface irrigation. ACKNOWLEDGMENTS This research was supported by and conducted at Arava Research and Development, “Arava” Experimental Station, Mobile Post Eilot 88820 Israel.

REFERENCES Camp, C.R. 1998. Subsurface drip irrigation: A review. Trans. ASAE 41:1353–1367.

1413

Carsel, R.F., and R.S. Parrish. 1988. Developing joint probability distributions of soil water retention characteristics. Water Resour. Res. 24:755–769. Gardner, W.R. 1958. Some steady state solutions to the unsaturated moisture flow equation with applications to evaporation from a water table. Soil Sci. 85:228–232. Lamm, F.R. 2002. Advantages and disadvantages of subsurface drip irrigation. Available at http://www.oznet.ksu.edu/sdi/Reports/2002/ ADofSDI.pdf (verified 26 Aug. 2004). Originally presented at International meeting on advances in drip/micro irrigation, Puerto de La Cruz, Tenerife, Canary Islands. 2–5 Dec. 2002. Kansas State University, Colby, KS. Lazarovitch, N., J. Simunek, and U. Shani. 2005. System dependent boundary conditions for water flow from a subsurface source. Soil Sci. Soc. Am. J. 69 (in press). Meshkat, M., R.C. Warner, and S.R. Workman. 2000. Evaporation reduction potential in an undisturbed soil irrigated with surface drip and sand tube irrigation. Trans. ASAE. 43:79–86. Philip, J.R. 1992. What happens near a quasi-linear point source? Water Resour. Res. 28:47–52. Shani, U., and D. Or. 1995. In situ method for estimating subsurface unsaturated hydraulic conductivity. Water Resour. Res. 21: 1863–1870. Shani, U., S. Xue, R. Gordin-Katz, and A.W. Warrick. 1996. Soillimiting discharge for subsurface emitters. I: Pressure measurements. J. Irrig. Drain. Eng. 122:291–295. Simunek, J., M. Sejna, and M.Th. van Genuchten. 1999. The HYDRUS-2D software package for simulating two-dimensional movement of water, heat, and multiple solutes in variably saturated media. Version 2.0. Rep. IGWMC-TPS-53. IGWMC, Colorado School of Mines, Golden, CO. Trooien, T.P., D.J. Hills, and F.R. Lamm. 2002. Drip irrigation with biological effluent. In Proc Irrigation Assn. Int’l Irrigation Technical Conf., 24–26 Oct. 2002, New Orleans, LA. Available at http:// www.oznet.ksu.edu/sdi/Reports/2002/DIBioEff.pdf (verified 26 Aug. 2004). Irrigation Assoc., Falls Church, VA. van Genuchten, M.Th. 1980. A closed-form equation for predicting the hydraulic conductivity of unsaturated soils. Soil Sci. Soc. Am. J. 44:892–898. Warrick, A.W. 1993. Unsaturated-saturated flow near a quasi-linear line source. Water Resour. Res. 29:3759–3762. Warrick, A.W., and U. Shani. 1996. Soil-limiting flow from subsurface emitters. II: Effect on uniformity. J. Irrig. Drain. Eng. 122:296–300.