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Soil Sensors vs ET

The Key Differences Between a Wireless Soil Moisture Sensor Network and Weather Based Evapotranspiration Systems for Controlling Irrigation – and Why You Should Consider Soil Sensor Technology

By Michael Parrish, Managing Partner

It is important to point out some significant differences between controlling irrigation using weather data versus controlling irrigation using soil data. We would be remiss if we didn’t take some time to highlight the differences. Essentially, using weather data and site variables to calculate evapotranspiration estimates in order to generate irrigation run times is like jumping in your car for a road trip, placing a piece of duct tape over your fuel gauge, and then using terrain, distance, speed, altitude, tire pressure and your car’s laboratory tested miles per gallon designation to estimate your fuel levels – it appears scientific at first glance, but would you trust this driving across the Nevada or Californian deserts with your young children in the car?  Or, you could simply use your fuel gauge for the actual measurement and know that you could make it safely to the next gas station? One is an estimate and one is an accurate measurement. Estimates can be useful; however, precision and maximum efficiency can only be achieved by accurate measurements. As the saying goes, you can’t manage what you can’t measure. So, key difference number one is that ET is an estimate and soil data is the actual measurement.

There seems to be a misconception that you can accomplish, on a zone by zone basis, the same results that utilizing soil data would produce by using ET hardware. Let us dispel that myth, as it is highly improbable.  The California Department of Water Resources states, on their California Irrigation Management Information System (CIMIS) website, that ET varies depending on plant (type, density, height) and soil factors and it is difficult, if not impossible, to measure weather parameters under all sets of conditions. This leads to key difference number two; utilizing ET moves you in the direction of greater complexity versus using wireless soil sensors to move in the direction of greater simplicity and maximized efficiency.

It is possible to use ET to arrive at useful estimates, but the chasm in efficiency between useful ET estimates and real time soil data is quite large. To even arrive at useful ET estimates a landscape manager needs to conduct a very thorough audit of the vegetative species in every station of irrigation. You need to document species types in every station, assign species factors in every station, assign density factors in every station, and assign microclimate factors in every station. Then you would need to determine if you are going to use crop coefficients or landscape coefficients, per station, when converting relative ET. For those stations that have more than one species type, you would need to decide which species factors and microclimate factors, within the categories of each, are most appropriate for your landscape quality goals, in every station. Areas near cars, sidewalks, concrete, courtyards and buildings need to be added into the factoring to determine which category for the landscape coefficient factors is the right one for each station.  Then, what is your existing irrigation efficiency station by station? Once that is all is determined, what is your starting point to begin the ET estimating process? In other words, what is your current soil moisture content at the moment that you begin your formulas? When there is a rain event what methodology would you have in place to determine the new starting point? Are you using historic weather data, off site weather data, or onsite weather data? Are you using relative ET and then converting it with a coefficient? If so, which stations would you use crop coefficient in and which stations would you use landscape coefficients in? Again, complexity versus simplicity.

By having real time soil data from the effective root zone, on a wireless sensor network in every station, our clients move away from the world of estimating with ET (not knowing) to a world of accurately measuring soil moisture (knowing). By being able to quickly determine the actual soil moisture level and water holding capacity of each station and then setting precise moisture targets for the adaptive watering algorithm, in response to that accurate data, our clients move away from complexity to simplicity.

In addition, our wireless soil sensor network will also give you soil temperature data and soil salinity data, both from the effective root zone of your landscape. That data will enable you to manage your landscape with far greater accuracy and efficiency, enabling you flush salinity from your soil when needed, and to prevent disease by applying chemicals and pre and post emergent at the optimal times.

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