The nitrogen in subsurface discharge and surface runoff was measured from four agricultural watersheds on Missouri Valley deep loess near Treynor, Iowa, from April 1969 through March 1974.The data showed that, with the agricultural management practices used on the watersheds, the subsurface discharge of water ranged from 62 to 88% of the average annual stream flow. Nitrate in subsurface discharge accounted for 84 to 95% of the total average annual soluble N discharged in stream flow.A terraced watershed continuously cropped to corn (Zea mays L.) had reduced surface runoff, sheet‐rill erosion, and associated nitrate‐nitrogen discharges, but had increased subsurface discharge of water and soluble N as compared with two contoured corn watersheds. Nitrogen fertilizer applied at a high rate (448 kg ha−1 year−1) exceeding crop needs on the terraced and contoured corn watersheds, increased five‐ and threefold the average annual subsurface discharge of NO3‐N, respectively, as compared with a contoured watershed fertilized with N at a normal rate (168 kg ha−1 year−1).To control the watershed discharge of N and subsequent pollution of stream flow from the Iowa and Missouri deep loess hills requires N fertilizer application rates that do not exceed crop needs and using conservation practices that minimize soil erosion and deep percolation.

R ESEARCH on three reservoirs in central Missouri has shown that reservoir sedimentation trap efficiency is affected by the detention time of storm runoff and bv factors governing FIG. 1 Progression of idow increments through reservoir. sediment particle size. ~e c r e a s i n~ the detention time can be done by discharging storm runoff from the reservoir with the use of a bottomwithdrawal spillway. With the bottom-withdrawal spillway, the clean water remains in the reservoir and floats above the density currents caused by storm runoff moving to the deepest part of the reservoir. This paper reviews previous research on TE and presents new findings on this important reservoir sedimentation factor. Some parameters that affect TE in three central Missouri reservoirs were evaluated and a new approach to control TE is described. In previous TE research, Brune (1953) found that the ratio of reser-the structure. Sometimes these estimates are very rough. These estimates, however, enter into the determinations of the capacity needed for sediment storage, total reservoir capacity, and the elevation of the principal spillway crest. Errors in any of these items may seriously affect the useful life and performance of the structure.

The authors [Wade and Heady, 1978] are to be commended for attempting to solve a most complex problem. The linear programing approach is a good technique for this problem.As the authors stated early in the paper, 'additional research would be required to establish the validity of the parameters of this model before specific policy recommendations could be made, since many of the parameters are estimated from limited data and by new procedures.' This statement, as well as errors in the paper restrict use of the paper to an illustration of what might be done.The comments in this letter are directed toward major points of contention in the paper, and follow the organization of the original paper.1. In the opening paragraph the authors stated that '... sediment is not easily controlled.' Actually, there is a wealth of information about controlling sediment; generally, the problem is doing it within the framework of a farmer's economics. We often have the knowledge, but not always the dollars. Besides construction costs, some erosion control systems, like terraces, are difficult to farm, particularly with large modern equipment. However, good land management is often more economical than traditional land management (such as no-till versus clean-tilled row crops) and is excellent for controlling erosion. Still another reason for lack of sediment control is that it takes an effort, and many farms suffer from benign neglect; farmers are simply too busy with the many other problems. Clearly, education is needed.2. There is, apparently, a symbol missing in (5), between XT and 0. 3. Under the section entitled 'erosion sector,' the authors stated that the '... analysis and verification of the procedures used to estimate erosion rates are a major part of this article.'The USLE was used to estimate sheet and rill erosion, and they developed guidelines for estimating delivery ratios and sediment transport ratios. These methods were described briefly, but the analysis escaped us, and certainly no verification was presented. 4. Figure I under system has an inconsistency which could be very important. The schematic shows that gully and channel erosion exists only from the nonvariable land use. To be used correctly, this box should be inserted before the sediment-delivery-ratio box, and then the erosion from the variable and nonvariable land-use boxes will input to this box of the schematic. The authors in this section also discussed.'... three distinct aspects of soil movement: Sources, delivery, and transport.' Most sedimentationists refer to these three components as erosion, transport, and deposition, or as upland soil displacement, delivery to channels, and transport in channels. We are not sure why the authors adopted their own concepts, rather than building upon accepted concepts.5. In the section on sediment delivery, the authors stated: 'Conventional procedures for computing delivery ratios proved inadequate...' The concept of sediment delivery ratios is admittedly crude, and delivery ratio relationships have not been de...

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The pace of research to understand erosion processes and to determine soil erosion rates and sediment yields has quickened in recent years. Many soil and water conservation planners have recognized the need to more fully understand the complex forces affecting detachment and movement of soil from erosion source to point of deposition. We learned long ago that deposited sediment limits the useful life of conservation structures and that we must optimize conservation designs because of a dwindling supply of good sites. More recently, with the advent of increased environmental concern, the necessity for the broadest consideration of all proposed developments and for predicting the environmental impact of optional soil and water conservation plans has been emphasized. The new awareness of the tremendous volume of urban and right‐of‐way construction erosion and its impact on society has intensified our concern. Furthermore, since sediment is a carrier of agricultural chemicals, the priority for a fuller understanding of soil erosion rates and watershed sediment yields has increased.