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MEMORANDUM December 13, 2013 Issues with Selecting an Appropriate Crop Growth Stage Coefficient for the SCS Mod. Blaney-Criddle Eqn. Page 4 of 34 PART II: FORENSIC ANALYSIS OF SCS TR-21 Introduction: In order to apply an elevation adjustment to the SCS Blaney-Criddle coefficients the elevation at which the curve was developed is needed. The SCS TR-21 publication provides crop growth stage coefficients for a variety of crops, but does not indicate where the curves were developed, or at what elevation. It has been my experience that within the water resources community in Colorado, no one knows where the coefficients were developed. I contacted two retired ARS researchers, Dr. Harold Duke10 and Dr. Marvin Jensen11, both are recognized as experts on crop water use and were working around the time SCS TR- 21 was published. Neither of the two knew the source of the coefficients12. Dr. Marvin Jensen mentions in his 2010 paper13 (herein referred to as “Jensen Paper”) that he worked under Dr. Harry Blaney in the 1960s. During my conversation with Dr. Jensen, he indicated that he is unaware of the work that the SCS14 did to develop the crop curves in TR-21 or where the data came from. He believes that the best source of where the data could have come from is found in ARS 127515. As part of my research, I also contacted Jerry Walker16 and Clarence Prestwich17 of the NRCS, both of whom are specialists within the agency in irrigation and water management. According to Mr. Walker, in the 1960s the SCS would only have relied on data provided by the ARS, a sister agency to the SCS18. Today, the NRCS will rely on University research and ARS work, but in prior years it was standard practice for the SCS to rely solely on ARS data. Mr. Walker indicated that any remaining SCS documentation on TR-21 would be located in Portland, Oregon at the West National Technology Support Center. Mr. Prestwich researched the SCS files in Portland and found a publication by ARS researchers Harry Blaney, Howard Haise, and Marvin Jensen entitled “Monthly Consumptive Use by Irrigated Crops in Western United States19” (herein referred to as “The Supplement”) and an internal SCS publication by Hyrum Woodward entitled “A Modification of the Blaney-Criddle Method for Computing Consumptive Use20” (herein referred to as “Woodward”). Woodward points out that monthly crop coefficients (k) used in the Blaney-Criddle equation were first shown in an internal SCS document, SCS-TP-96, and later published in ARS 1275, a revision of SCS-TP- 96. The Supplement is a provisional supplement to SCS-TP-96 that associates the data to ARS researchers from the Western United States. Included in the document are k values and information on specific years when data were collected. It is interesting to note that the data in The Supplement don’t quite match up with the ARS 1275 data, which suggests that there must have been some adjustment by the authors (notably H. Blaney21) between publications. Woodward also mentions that the kc values were taken from smoothed curves plotted from available measured consumptive use data. The smooth curves were drawn thru plotted points with minor adjustments made so that the sum of the computed monthly values would approximate the seasonal values developed for the original Blaney-Criddle equation. Woodward describes the procedure used for determining the location of the end dates, which was done by “laying a straight edge across the curve between the end points and balancing visually the areas between the straight edge and the curve.” It is important to note that Woodward points out that kc values for some crops were estimated since no measured consumptive use values were available and data from similar crops were used to “guide the estimator.” Since learning that the source of the data used in SCS TR-21 was ARS 1275, we should be able to identify what plant species the generalized crop curves represent, where the data was developed, and at 10 Category I (lead) Scientist from1967-2002, USDA-ARS-WMU 11 Category I (lead) Scientist from1955-1987. Dr. Jensen was inducted in to the ARS Hall of Fame in 2000 12 Dr. Duke did mention that data were collected from around the Western US primarily from soil core samples 13 Jensen (2010) 14 In 1994 the SCS became the NRCS 15 Blaney & Criddle (1962), Table 18, pp.49-52 16 Irrig. Engr., Central National Techn. Support Cntr., Ft Worth, TX and a primary reviewer of the NRCS NEH Part 652 – Irrig. Guide 17 Water Management Engr., W. National Techn. Support Cntr., Portland, OR 18 Today the SCS and ARS are sister agencies within the USDA. During my conversation with Dr. Harold Duke he indicated that in the 1950s, and prior to, ARS was a branch of the SCS 19 Blaney, et al. (unk) 20 Woodward (1963) 21 Dr. Harry Blaney is the only author common to both ARS 1275 and The Supplement