Location: Soil and Water Management ResearchTitle: Soil moisture monitoring methods:Strengths and limitations
Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: 2/2/2015
Publication Date: 2/2/2015
Citation: Evett, S.R. 2015. Soil moisture monitoring methods:Strengths and limitations [abstract.Webinar Series.
Technical Abstract: Equipment for monitoring soil water content and sometimes bulk electrical conductivity can be used for scheduling irrigations if the accuracy of the equipment is sufficient to avoid damanging plants and wasting water and fertilizer. Irrigation scheduling is the process of deciding when to irrigate and how much to apply. Its goals are maximum profitability balanced against sustainability. Objectives include avoiding, limiting or applying stress to plants depending on the crop and time in the cropping cycle. Other objectives include leaching a field of salts, avoiding deep leaching to improve fertilizer use efficiency, preparation for planting, etc. Soil water monitoring methods include the Neutron probe (NP), Time domain methods (TDR and TDT), Capacitance and other frequency domain (FD) methods and Gravimetric sampling. Usually, soil water monitoring is effective only between the field capacity soil water content and the water content at wilting point. The range between these limits can be as small as 0.04 m3 m-3 to as large as 0.21 m3 m-3, but the effective range is reduced further due to the need to avoid allowing the soil to dry to the wilting point at which point the crop would be permanently damaged. This maximum allowed depletion of soil water reduces the range within which soil water sensors must work to a minimum of 0.02 m3 m-3 to as large as 0.13 m3 m-3. Soil water sensors must be accurate to within 0.02 m3 m-3 to avoid damaging plants during irrigation scheduling in most soils. Many soil water sensors were studied to find if they were suitably accurate. All soil water content sensors require soil-specific calibration – particularly capacitance sensors and most time domain transmission (TDT) sensors. Electromagnetic fields from capacitance sensors do not uniformly interrogate the soil – leading to unrealistic spatial variation of water content. Interference from bulk electrical conductivity and bound water effects are not corrected in most EM sensors. Irrigation scheduling & ET estimation using the capacitance type EM sensors is problematic due to these effects. Therefore the capacitance sensors are not recommended for irrigation scheduling. Two new sensors based on time domain measurements were found suitably accurate and inexpensive enough to be widely used. In addition, the granular matrix sensors are useful for some crops and soils, particularly the more shallow rooted vegetable crops grown in loamy and silt loam soils.