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22 Nov. 1996 Dear Dean and Sowacs: Dean, your "original" question was something like this -- "someone said that neutron probes don't measure ET," and then you asked "isn't that also true for TDR?". I hope we agree that the statement about neutron probes is accepted as true, and your question about TDR is also "yes". But both TDR and neutron probes can be useful in determining water balance parameters, and then if all the water balance components can be characterized spatially and temporally then ET can be determined. This certainly not a trivial matter nor is it a simple one! That's why we have jobs working on these issues! Your latest discussion and questions about the "practical" side and the "money" side of the issue are critical issues. As you know, you are near some of the world's foremost authorities on water use in the Univ. of Calif. system (UCD, UCR, UCB, ...), other state univ. (Cal. Poly, CSU-Fresno, etc.), and even Federal agencies (ARS, NRCS, BR, ......). The California agricultural issue is complicated by the diversity of its agriculture. When I was working in Fresno, I think there were something like 240 crops being commercially produced in Fresno county alone! I do have a high personal regard for the "Bible" on Calif. water use (DWR 113-3). I don't think you can go too far wrong revising and updating those values based on the improved climatic information from CIMIS now available that wasn't available when 113-3 and 113-4 were written. I do think that Bowen ratio and eddy correlation tools may need to be added to your DWR arsenal of weapons along with sap flow gauges and TDR to supplement weather data and neutron probe data. The problem is that someone (or more accurately a team) needs specific training and coaching in using each of these tools. If I had to make TDR measurements, I'd be lost, but I've got Steve Evett next door who knows how to do it. If I had to build a Bowen ratio system from scratch, I could but it might look funny (as many I've built look), but I can call or visit with experts (Lloyd Gay, Leo Fritschen, or with CSI or REBS). I might be able to build a sap flow gauge too, but I'm way ahead to buy one (Dynamax send help!!!) but then I may need some hand-holding to get it working. My experience has taught me over the years, that nothing is simple or even correct as it comes out the box and user's manual never explain all the problems. Back to the "money" and water saved issue! I've gotten many "side" messages from folks during these exchanges. One is very appropriate here -- [from Ron Elliott, Oklahoma State Univ.] "How was cooper wire invented? By two lawyers pulling on a penny!" The lawyers are going to get rich and all we get are headaches! The water transfer issue is very complicated. I hate to get into this at all, because it so challenging. But it's Friday and I'll be gone all next week so my mail box can fill up without me reading it! You highlighted the issue -- "real water" versus just water. Only water consumptively used in a closed basin can be sold -- that's real water. Otherwise any excess applied water becomes someone else's water supply. In my state (Texas), there's absolutely no hope of knowing this!!!! But I hold out hope that Californians before many others can get a good handle on this problem. As you indicated it is REAL, and it has important SOCIAL and ECONOMIC consequences. I'm sure you will get many enlightened responses from the SOWACS group on how to best proceed on tackling these issues! Terry Howell ********************************************************************* * Terry A. Howell, Ph.D., P.E. (806) 356-5746 * * USDA-ARS (806) 356-5750 (Fax) * * P.O. Drawer 10 tahowell@ag.gov (E-mail) * * Bushland, TX 79012 http://www.net.usda.gov/cprl/ (Internet) * * * * 1/2 mi. West I-40 South Access Rd. (shipping) * *********************************************************************