Role of Groundwater Flow in Tile Drain Discharge Peter J. Vaughan,* Donald L. Suarez, Jirka Simunek, Dennis L. Corwin, and James D. Rhoades ABSTRACT ILE DRAINAGE in agricultural areas provides a Tile systems drain water applied to agricultural fields as irrigation Tmethod of regulating the depth of shallow water and precipitation but also may intercept regional groundwater flow. tables. The water drained from tile systems is frequently Identification and characterization of the potential sources of tile not reusable as irrigation water because of high salinity water is essential for informed management of salinity and contami- and contamination. Two contaminants often present in nants. Factors influencing tile discharge including depth of water ap- the San Joaquin Valley of California are B and Se. plied, evapotranspiration, water storage, drain blockage, and intercep- Disposal of tile water in this area is a necessary compo- tion of regional groundwater flow were evaluated to determine which nent of a management system that includes tile drainage. may be related to a fivefold variation in cumulative tile discharge among six sumps located 100 km west of Fresno, CA. Cumulative But tile water disposal raises environmental concerns depths drained were calculated for 5 yr of weekly irrigation, precipita- such as the well-known experience of Se contamination tion, and discharge data. Evapotranspiration and water storage were at Kesterson reservoir in the San Joaquin Valley. The estimated using the UnsatchemGeo variably-saturated water flow source of tile water is shallow groundwater which may model. Well water levels measured on 19 dates were spatially-averaged be derived from water applied during irrigation of a providing spatial variation of depth-to- water among the drained areas. field, from local precipitation, or it may have moved Variability in depth of water drained (0.18-0.95 m) was large and was laterally to the tile drain by regional groundwater flow. not correlated with either water applied (3.26-4.58 m, r2 = 0.03) or The determination of the relative importance of these with computed water flux from the bottom of the soil column (0.05- components is significant because drainage of irrigation 0.31 m, r2 = 0.00). Groundwater interception by tile drains was a water represents an economic loss. Drainage of ground- factor because depth-to-water was negatively correlated with discharge 2 water which has moved laterally from other areas is, (r = 0.42) and drawdown of groundwater levels by drains was rela- however, not locally controllable and cannot be factored tively larger for those drained areas encountered first during regional groundwater flow. For all six sumps, drained water is likely derived into the local economics of production. Regulation of from locally applied water and interception of regional groundwater drain water quality is therefore a complex issue because flow implying that standard two-dimensional models of flow to drains, there are, potentially both local and regional compo- representing only water applied locally, would not be applicable to nents controlling water quality. modeling of drain flows or drain-water solute concentrations. Tile drain tubes are normally installed in parallel sets with a fixed spacing. This arrangement can be modeled by analytical techniques which lead to expressions for U.S. Salinity Laboratory, 450 W. Big Springs Road, Riverside, CA flow rates within tubes based on drain diameter, spacing 92507. Received 18 Nov. 1997. *Corresponding author (pvaughan@ ussl.ars.usda.gov). Abbreviations: CIMIS, California Irrigation Management Informa- Published in J. Environ. Qual. 28:403-410 (1999). tion System; CIS, geographic information system. 404 J. ENVIRON. QUAL., VOL 28, MARCH-APRIL 1999 and various boundary conditions for water flux and hy- These geochemical studies of groundwater and tile- draulic head (Kirkham, 1949, 1958; Jury, 1975a; Fipps drained water on the west side of the San Joaquin Valley and Skaggs, 1991). The lower boundary condition of indicated that a significant portion of the water ap- these models is an impervious layer which permits a pearing in tile drains was not applied to the same field, solution using the method of images. This assumed but was water that had flowed upwards from greater boundaryimplies that all water arriving at a drain origi- depths and horizontally from the west. nated at the soil surface between the drains. In the In this study, we examined the water budget of an analytical solutions, certain flow paths pass through irrigated agricultural area located in the same general lower elevations than the elevation of the drain resulting area of the western San Joaquin Valley as the aforemen- in upwards flow to the drains. Such paths may have tioned groundwater geochemical studies (Deverel and travel times on the order of 10 yr or more (Jury, 1975b). Gallanthine, 1989; Deverel and Fio, 1991). Considering The utility of the impervious layer assumption is ques- Wet-]- I42ro ~- Wdp~- AWst-I- Wtd-- Wip-- Wig= 0 [1] tionable under certain field conditions as, for example, in the western San Joaquin Valley. In this area, a highly a water balance in terms of water depth ILl for a control permeable sand layer underlies clay-rich soils that have volume consisting of a soil column and a tile drain. much lower permeability (Deverel and Gallanthine, Water enters the volume as irrigation, precipitation or 1989; Belitz and Phillips, 1995). Groundwater flow is tile interception of groundwater and leaves as evapo- from southwest to northeast and the historic record of transpiration, runoff, deep percolation, and tile drain- artesian wells at the margin of the alluvial fan near the age. The terms are: w~p depth of water applied through San Joaquin River (Mendenhall et al., 1916) suggested irrigation and precipitation, wet evapotranspiration, Wro that this groundwater flow was occurring in conditions runoff, Wdpdeep percolation, w,d tile drainage, Awst where the groundwater level was close to the surface. change in soil water storage (expressed as a depth of Groundwater levels in this area were lowered by pump- water within a soil column of specified height), and W~g ing during the first half of the 20th century, but the groundwaterinterception by tile drains. Spatial variabil- introduction of irrigation water from northern Califor- ity in the depth of water drained by tile systems could nia, through the Delta-Mendota canal, and the installa- be caused by spatial variability in any of the amounts tion of tile drainage have again stabilized the water table represented in Eq. [1] or by some combination these at a depth of 1.5 to 3 m (Deverel and Gallanthine, 1989). terms. The question addressed here was the origin of Water flowing to tile drains in this area is a mixture of variability in depth of water drained given these spa- local irrigation water and water that has movedupwards tially-variable processes. from the sand layer (Deverel and Fujii, 1988). Evidence for this partitioning includes isotopic enrichment of g180 MATERIALS AND METHODS with depth, and decreases in tritium concentrations Field Area when compared with irrigation water sampled at the same site (Deverel and Fio, 1991). Based on water chem- The BroadviewWater District is located on the west side of the San Joaquin Valley of California approximately 100 istry data, the proportion of drained water that arrived kmwest of Fresno. Mostof the water district is divided into from greater depths was 30%for a drain at 1.8-m depth quarter-sections whichare square fields with area of 64.8 ha. and 60%for a 2.7-m drain. Furthermore, Se concentra- Onthe borders of the waterdistrict there are also somesmaller tions indicated that the deep groundwater component fields with irregular shapes. This study was concernedwith had arrived from the west (Deverel and Fio, 1991). only quarter-sections, and henceforthwe will refer to quarter- sections as fields. Thedistrict maintainsirrigation turnouts in 4-2 2 the southwest corner of each field. Watersupplied fromthese ¯ Sumps turnouts is meteredwith a precision of 123 m3 (0.1 acre-feet). (numbered) Sumpsare located in the northeast corner of certain fields for o Wells collection and disposal of waterfrom tile-drain systems. Water [~ Tile Drains draining into the sumpsis pumpedinto drainage ditches and metered to the same precision as the irrigation water. The ~ Drainage Areas sumpsmay drain a single field, two adjacent fields, or all four StudyArea 10-2 ~ fields that makeup an entire section (Fig. 1). Becausesumps Boundary maydrain morethan one field, the term drainagearea is used to refer to the entire area drained by a sump. /~ Groundwater Movement Approximately80% of the water district has tile drainage at approximately 1.8-m depth. The most commonspacing of drain laterals is 91.4 m, but several other spacings are also 16, present (Fig. 1). Mapsof drains were compiledby the water i15"~ I 1 km I N district staff using data obtainedfrom individual landowners. Six sumpswere selected for analysis of irrigation and drainage, each located in the northeast corner of an entire section. Every 2 d, cumulativeflow meters for irrigation were read Fig. 1. Map of six drainage areas that drain to the numbered sump in the northeast corner of each area. Groundwater movement direc- and weekly readings of meters on sumpswere taken during tion was a mean value computed for the entire water district using the period of i Jan. 1991 to 1 May1996 (the study period). water table elevation data for 1991. Well locations are 24 of the Daily precipitation data, collected at the California Irrigation 57 wells used in water table surface calculations. ManagementInformation System (CIMIS)station 10 km east VAUGttANET AL.: ROLEOF GROUNDWATERFLOW IN TILE DRAINDISCHARGE 405 of the study area, in Firebaugh, were combinedwith the irriga- q = -A exp(-Blh - h0[) [2] tion data to generate a complete record of water applied to 3 -2 3 -~ -Z] each field.