Project Summary   
Water resources management in California is at a critical stage.  It is imperative that water losses be minimized where ever possible.  A potentially large problem is seepage from unlined (earthen) ditches, canals, and reservoirs.  In the past this may not have been deemed critical since in many cases, the losses would flow back to an aquifer for later pumping.  However, some of this loss would flow to a salt sink and thus become unusable.  In addition, non point source pollution issues have become important.  And, as some water that had been used by agriculture has been reallocated to other users, it becomes more important that the full diversion reaches its destination.

Addressing the problem of seepage involves five major phases:

      1.      Locating the seepage

2.      Quantifying the seepage  

3.      Identifying the available solutions

4.      Determining the economics of the available solutions

5.      Implementing the best solution

Another factor involved may be an assessment of impacts from seepage apart from the value of water lost.

However, given a significant problem and given a set budget, it is desirable that the most money possible should be available for implementing the solution.  Thus, it is desirable that the least-expensive method for locating and quantifying seepage be used, consistent with acceptable accuracy.  

Project Background: A defined Best Management Practice (BMP) for Central Valley Project (CVP) districts is Distribution System Lining/Piping.  A draft Methodology for Determining Applicability/Implementation of the BMP identified “Estimate of Conveyance Losses” as step 2 (after an assessment of current implementation level) of the determination.  The assessment of applicability for any BMP is not intended to be onerous.  Thus, identification of fast and inexpensive methods for the initial estimate of conveyance losses is desirable.

Methods for identifying and quantifying seepage include:

• Visual and standard measurements indicating losses in one area or another- possibly indicated by swampy areas, ponding water, excessive vegetation

• Ponding tests – whereby a section of channel is blocked off, filled with water, and then observed closely to measure the rate at which the water level decreases

• Inflow-Outflow tests – whereby the inflow and outflow in a channel section is accurately measured.  Any difference is attributed to seepage (after evaporation losses are accounted for)

• Electromagnetic-inductance surveys – whereby the electro-magnetic properties of the channel are measured to some depth.  Differences in readings are then correlated to known areas of seepage.

• Seepage meters – these are basically miniature ponding tests whereby an enclosure is placed over a very small area of the channel and the seepage from this enclosed area measured.

Each of the above methods has its advantages and disadvantages. 

One of the fastest methods for surveying channels is electromagnetic-inductance surveys (EM).  Basically, this type of instrument generates an alternating magnetic field in the ground, which in turn causes eddy currents to flow.  The strength of these currents is a function of the apparent ground conductivity.  

The apparent ground conductivity is dependent on a number of factors.  The most important of these are the clay content of the soil, soil and soil-moisture salinity levels, and total soil moisture.  The higher the clay content, soil moisture level, or soil/soil-moisture salinity, the higher the apparent ground conductivity measured via EM methods.  Thus, in the context of an EM survey of an earthen conveyance channel, lower relative conductivity readings would indicate lighter soils (sand) and lower salinity levels (possibly leached areas), either of which might indicate potential seepage problems.

A key factor in the use of EM surveys for environmental assessments is the calibration of the results.  The EM survey data is relative data.  Physical investigations must be performed to calibrate the readings so that absolute conditions can be estimated.  In the context of a seepage investigation this might include sampling to verify sandier sections, or ponding or seepage meter tests.  These targeted physical investigations are done based on a statistical analysis of the raw EM survey data.

The targeted calibration data, in conjunction with the survey data, is then used to develop an assessment of the total area (or length of channel in this context) covered by the survey.  

Project Components: There are four major components to this project

1. The instrument platform.  This is a Spra-Coupe model 3640 similar to the one seen in Figure 1.  This vehicle provides the following:

• enclosed, air conditioned, dust-free environment for the GPS receiver and computer
• variable wheel-base for use in furrowed fields
• enough weight so that a mounted soil sampler will operate successfully
• automatic transmission for smooth operation
• a full hydraulics system that will be used in mounting the electromagnetic instruments

Figure 1 - The salinity assessment vehicle at Imperial Irrigation District - the planned Seepage Assessment Vehicle will be similar - the S-shaped pipe at the vehicle's rear rotates down so as to drag the EM 38 instrument pack (seen in Figure 2 below)

Figure 2 - EM 38 instrument pack.  This is dragged behind the vehicle in Figure 1 to take readings of soil salinity.  Note that the instrument must be at least 6 feet from any metal.  Thus, the mounting is PVC pipe.

2. The GPS unit.  This is a standard Trimble Ag 132.  We will be subscribing to a satellite differental correction system.
   
3. The electromagnetic instrument.  This will be a Geonics EM31.  This instrument will provide readings from about 20 feet deep with the coils in verticle orientation, from about 10 feet in horizontal mode.  We also will have access to an EM 38 (seen in Figure 2) through a cooperative agreement with the local USDA Agricultural Research Service and Dr. Tom Trout.
   

4. Software.  This will include some datalogging software from Sandia Research.  Their software basically "marries" any instrument readings with the GPS data.  It also provides the guidance to steer back to any location in order to perform physical investigations for calibration.  Another piece of software has been developed by the USDA Salinity Research Laboratory located at UC Riverside.  This software performs a statistical analysis of the raw data, identifying the different "types" of data received.  It then develops a sampling protocol which will allow calibration of the full data set.

Groundtruthing: The most important part of this project will be developing the protocol for calibrating the data.  This is sometimes known as groundtruthing.  Physical investigations (soil sampling, infiltrometer tests, lab chemical analyses, etc.) must be performed in order to calibrate the data.  We will be forming a technical advisory committee to help in the development.

A feature of the vehicle will be a mounted soil sampling unit (as in Figure 3).

Figure 3 - Mounted soil sampler on the IID Salinity Assessment Vehicle (see Figure 1)

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Page last updated 12/20/03     -    Comments/Questions - PCanessa@charter.et