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~'-I ANALYSIS OF SALT BALANCE -.-:.~ "' :;?j AND SALT-BURDEN DATA ON THE RIO GRANDE by L. V. WILCOX Assistant Director, Salinity Laboratory, Agricultural Research Service, U. S. Department of Agriculture, Riverside, California .,-; ~:j 7'":iJ :::j , ~ -'--1 The Rio Grande is one of the more important streams of the add West. Essentially all of the water is diverted and most of it is used for irrigation. Agriculture issuccessfulwhere drainage is adequate, but where drainage is impaired, salinity conditions restrict crop growth. These problems were recog- . nized more than 20 fears ago when the present in- vestigation was started. This investigation has been a cooperative pwject between the United States Section of the International Boundary and Water Commission, the U. S. Bureau of Reclamation and the U. S:Geological Swvey of the Department of Intedm, and the U. S. Salinity Laboratory of the U. S. De- partment of Agriculture. This paper summarizes the findings of certain phases of the study to 1955, but it is not a final report. :;:1 "J ~ DEFINITION OF TERMS Salt balance is defined as the relationship between the quantity of dissolved salts carried to an area in irrigation water and the quantity of dissolved salts removed by the dminage water. Scofield' odginated the term in the statement: "If the mass of the salt input exceeds the mass of the salt output, the salt balance is regarded as adverse, beca use this trend is in the direction of the accumulation .of salt in the area and such a trend is manifestly undesirable." The results are expressed as quantities of salts or in terms of percentages as "Output as percent of Input" . Salt burden is the quantity of salt passing a sta- tion on a stream in a given period of time. It is usually expressed as tons or ton-equivalents per year. J METHODS AND ACCURACY The basic data of this investigation include meas- urements of stream flow and the chemical analyses of related water samples. Discharge measurements were made at seven gaging stations on the main stem IS cofield, Carl 5., 1940. Salt Balance in Irril1ated Areas. ]oumal of Agr. Res. 61: 17~39. of the Rio Grande and on several -irrigation and drainage canals in the Rio Grande Project. The measurements were made by engineers of the co- operating agencies. "Provisional Records" were furnished cunently and ahead of publication and are the values used in this study. The water samples were collected at the gaging stations by the engineers who made the discharge measurements. During the early years of the in- vestigation, all of the water samples were analyzed by the Salinity Laboratory. Since 1946, the samples from Otowi Bridge and San Marcial have been ana~ lyzed by the U. S. Geological Survey. The samples analyzed by the Salinity Laboratory were composited on the basis of the discharge of the stream. Those analyzed by the Geological Survey were composited on the basis of equal aliquots, but in both cases the total volume of water represented by each sample is known. The salt-burden and salt-balance conditions have been calculated from these discharge measure- ments and chemical analyses. a The "Provisional Records" of discharge that have been used may vary slightly from the final pub- lished figures. For purposes of statistical analysis, these values have been used in terms of Uthousands of acre feet" which reduces the number of signifi- cant figures to three or four. The chemical analyses are reported to four significant figures. It is be- lieved that final results should be accurate to three significant figures. RESULTS SALT BURDEN The locations of the seven gaging stations on the Rio Grande are shown in Figure 1. The station at Otowi 8ridge is above the irrigated area in New Mexico. The San Luis Valley in Colorado is above this point but there is no return flow of drainage water. The Middle Rio Grande Valley Pwject .is 2Discharge measurements and some of the water analyses are published in the Water Supply Papers of the U. S. Geological Surveyor in the Water Bulletins of the International Boundary and Water Commission. 39 .-\. r.-' ,.. .r t, .~I':;; L t.l i1