flooded throughout the growing season; 2.36 ac-ftlac were lost by seepage.) Surface irrigation the major crops in PDD are tomato, cot- Although the concentration of TDS ton, and other row and field crops, which in the spill water increased 1.7-foId, the return flowsvary are normally furrow irrigated. unit mass emission rate (Iblac) was only GCID captures and reuses about 44 44 percent of that brought in by the sup- percent of its drain waters within the dis- ply water, which is in line with the dis- trict and discharges the rest into the trict-wide 61 percent mass emission of Kenneth K. Tanji Colusa Basin Drain (CBD), where it is re- salts. Note that concentration of sus- Muhammad M. lqbal used downstream before final discharge pended solids was reduced by about one- Ann F. Quek back into the Sacramento River. PDD half, and the mass by nearly one-tenth. Ronald M.Van De Pol and the neighboring Central California As for nutrients, the nitrogen con- Linda P. Wagenet Irrigation District capture and reuse centration in the drain water was about Roger Fujii about 20 percent of PDDs surface drain- 1% times greater, but only 38 percent of Rudy J. Schnagl age (not counting reuse at farm levels) the nitrogen brought in by the water was Dave A. Prewitt and discharge the remainder into the discharged. It should be noted that about Grasslands Water District, where it is re- 90 percent of the total nitrogen was in uch attention is being focused on used in irrigated pastures and waterfowl the form of organic nitrogen, which im- M irrigation return flows as a result habitats. A small part eventually enters plies only small losses of chemical nitro- of recent legislation on water quality and the middle reaches of the San Joaquin gen fertilizers in runoff waters. pollution control and the concern for River. Table 4 contains water quantity water and energy conservation. State- Table 2 presents the flow-weighted and quality data for a tile-drained farm wide, surface irrigation return flows are average quality of supply and drain cropped to cotton, tomato, and wheat in nearly nonexistent where water normally waters from CGID and PDD. The impli- the PDD. The grower blended drain wa- is scarce or expensive. This report des- cations of these data are of considerable ters captured from the Panoche Drain cribes the variations in flow and quality importance. In CGID, the 2-fold increase with fresh water obtained from the Del- characteristics of surface drainage waters in concentration of total dissolved solids ta-Mendota Canal (DMC). Quality para- from two irrigation districts in Cali- (TDS)relative to supply water can be at- meters are given for the waters and for fornia’s Central Valley, and the factors tributed to the usual 2- to 4-fold increase the tile effluents collected in two tile that contribute to such variations. by evapotranspiration by crop plants. In sumps. The extremely high salinity level, PDD, however, the increase is 9.6-fold high boron content, and low sediment Irrigation return flows and quality and must be attributed to the pickup of concentration of the tile effluents are due characteristics salts from the chemical weathering (dis- more to the dissolved salts, gypsum, and solution) of native soil salts and minerals boron native to the soil than they are to The major components of surface (primarily gypsum). This is confirmed by the supply water. return flows include surface runoff (irri- soil and tile drain water analyses. A computer model predicted the gation tail water, rainfall runoff, and op- The increase in concentration of total dissolved salts of tile effluents in erational spills from distribution systems) suspended solids (SS)in drain water rela- PDD would be 7,150 mghter, which and collected subsurface drainage (efflu- tive to supply water is 1.5-fold in GCID, agrees with the 6,700 to 8,900 mgfliter ents from tile drainage and drainage and again much larger (3.9-fold) in PDD. (flow-weighted average of 7,380 mglliter) wells, and subsurface waters intercepted This may be attributed to two major fac- reported in table 4. The model predicted by natural and man-made open channels). tors: (1)the flooded rice fields of GCID the concentration of total dissolved solids With some exceptions, the various tend to act as settling basins for sus- in the surface irrigation return flow would components of surface return flows are pended matter, and (2) because of their be 1,820 mglliter (and that it would have collected in the same drain regardless of physical and chemical properties, surface been only 460 mg/liter if gypsum were their origin (surface or subsurface, point soils in the PDD are more susceptible to not present in the PDD soils). The mea- source or nonpoint source). A part of these erosion under surface irrigation methods. sured concentration was 2,050 mgfliter drain waters is reused, either by plan or However, on a mass basis, only (table 21. incidentally, at the site of production or slightly more sediments were discharged In summary, the ranges of varia- downstream. The remainder is discharged by PDD than were brought in by the tions in the quality and quantity of sur- with no readily apparent beneficial uses. supply water (15,487 vs. 13,135 tons). The face irrigation return flows are highly Table 1 shows expected differences impact of pollutants on possibilities of site-specific, and are affected by the para- in quality characteristics for collected water use often is appraised in terms of meters presented here and by many irrigation return flows. These are des- concentrations only; mass emission of other factors. Quantity is influenced by: cribed relative to the supply water, be- pollutants also should be considered. availability and cost of supply water; ir- cause actual concentrations in both supply rigation application methods and ef- and discharge waters are highly variable Rice fields vs. tile-drained farm ficiencies; extent of reuse at the on-farm, from place to place. district, and basin levels; special cultural Table 3 summarizes data obtained practices; and constraints on reuse due District supply water vs. drain from four commercial rice fields in GCID. to the presence of excess boron, sodium, water The supply (inflow) and the surface runoff and chloride, Quality is influenced by: (outflow) waters were continuously moni- the supply water; presence of salts, Figure 1 is a schematic diagram of tored over the whole growing season, and boron, and nitrogen native to the soils; supply and drain waters for the Glenn- various quality parameters were mea- leaching fraction and salt pickuplsalt Colusa Irrigation District (GCID) in the sured at weekly to twice-monthly inter- deposition phenomenon; use of agricul- west side of the Sacramento Valley; fig- vals. The average amount of water ap- tural chemicals and wastes, such as ani- ure 2 is for the Panoche Drainage Dis- plied was 7.64 acre-feet per acre (ac-ftlac), mal manures; erodibility of surface soils trict (PDDI in the west side of the San of which 1.91 ac-ft/ac were discharged as and open drain channel banks; and dis- Joaquin Valley. The dominant crop in spill water. (An estimated 3.37 ac-ft/ac charges into irrigation drains by other GCID is rice, which is usually contour were used in evapotranspiration, and sectors of society. 30CWWIAAGRICULTURE. MAY 1977 Kenneth K. Tanji is Associate Water Sci- R. J. Miller and W. 0. Pruitt of the same gation was made possible through the entist and Principal Investigator, Mu- department and G. L. Horner of USDA- cooperation and support of Robert Clark hammad M. Iqbal is Junior Developmen- Economic Research Service. We also ac- and Richard Haupala of Glenn-Colusa Irri- tal Engineer, Ann F. Quek is Staff Re- knowledge the assistance of J. W. Gold- gation District and Ray Ram of Panoche search Associate III, Ronald M. Van De inger, D. W. Wolfe, and J. A. Sellick for Drainage District. This work was partly Pol was Postgraduate Research Water laboratory and field work This investi- supported by EPA Grant No. R803603. Scientist, Linda P. Wagenet was Re- search Assistant, Roger Fujii and Rudy J. Schnagl are Research Assistants, and Dave A. Prewitt is Staff Research Associ- ate I, all of the Department of Land, Air, and Water Resources, University of Cali- fornia, Davis. This project staff acknow- ledges the overall guidance and support given by Co-Investigators J. W. Biggar, TABLE 1 Collected Surface Irrigation Return Flow Components and Their Expected Qualily Characteristics as Related to Applied Wales 835.400 ac-It 131,793 tn TDS Operational lrrig Subsurface Quality parameters rPllls tail water drainage 27,267 tn SS General quality 0 + ++ Salinity 0 o+ ++ Nitrogen 0 o+ ++ ++ + Oxygen demanding organics 0 +o 0-- Sediments o+ - ++ -- 6 31 ac-ft/ac Pesticide residues 0 ++ 0-+ 1 00 Wac TDS P hosp hog us 0 ++ 0- + 0 21 Wac SS 0 = not expected to be much different than supply water Recapture + - = some slight increaselpichup of decreaseldeposition n may occut 499,700 ac-ft + + = usually expected to be significantly higher due to concentrating effects application of agricultural 'I chemicals erosional losses pickup of natural geochemical 243,500 ac4t sources etc 80.803 tn TDS -- = usually expected to be significantly lower due to 11,922 tn SS liltration fixation microbial degradation etc TABLE 2 Quality of Supply and Surface Drain Waters in Glenn Colusa Irrigation District IGCID) and Panoche Drainage District IPDDJ for the 1975 Irrigation Season 1 84 ac-ft/ac 0 61 Wac TDS Supply water Drain water 0 09 Wac SS Qiial ty Parameter GrlD PDD GClD PDD Elerliicnl conductlvltvltri nilrmmlios cm I80 363 391 3 070 Flg.