Soil Moisture Measurement for Irrigation Scheduling Sanjay Shukla Agricultural and Biological Engineering UF-IFAS

Irrigation Scheduling Appropriate water at appropriate time Crop need, soil properties, and weather condition Different methods Experience Calendar method (0.8 in every 4 th day) Soil water measurement based irrigation Dynamic water balance – water budget

Soil Moisture Measurement Direct method Gravimetric method Indirect Tensiometric ( energy status – related to moisture) Tensiometers Resistance blocks Psycrometer… Volumetric Nuclear method (Neutron probe) Dielectric methods Time Domain Reflectometry (TDR) Frequency Domain Reflectometry (FDR) Capacitance, TDT, ADR, Phase Transmission Other [After EDIS Bul. AE266]

Types of Devices Fixed measurements at a fixed location depths single (e.g. 8 in.) multiple (e.g. 4, 8, 12 in.) Portable fixed location access pipes with portable reader Hand held can measure moisture anywhere at farm/grove

BREAK / DEMO

Tensiometer Tensiometers Water potential Need to related tension to volumetric water content to know available water Soil water characteristics curve (Put a figure for a variety of soils) Advantages Relatively inexpensive ($100) and easy to use Limitations Slow response, needs maintenance, manual reading, lack of contact in sandy soils

Electrical Resistance Porous Blocks to measure electrical resistance as a function of water content Advantages Low cost and maintenance Ease of use Limitations Delayed response in sandy soils Dry conditions - reinstallation Errors in soils with high salinity

Dielectric Method Based on medium’s capacity (dielectric constant) to transmit high frequency electromagnetic wave/frequency D Const = 1 for air, 2-5 for soil, and 80 for water Two approaches Time Domain Reflectrometry (TDR) Frequency Domain Reflectometry (FDR) Better devices than other types More costly than tensiometer and resistance types

Capacitance Probe Capacitance probe Dielectric properties of soil Modern probes can log data for every 5 minute and higher Single and multiple depth probes Manual measurements to continuous logging Manual: Diviner, Echo Probe Automatic: Enviroscan, EasyAG, C Probe Accurate measurement Data can be stored for 2-4 weeks and downloaded to computer

Examples of Types of Devices

Capacitance Probe

Portable Capacitance Probe Cost = $2000

TDR Probes Estimate the dielectric constant by the travel time for electromagnetic wave to go through a transmission line (parallel rods) Measure average soil moisture content along the waveguide (soil cylinder of approx 1.5 times the spacing) Better accuracy Types of devices Portable: e.g. Hydrosense ($600) Automatic: e.g. CS 616 (Campbell Sci.) Can be used to measure moisture at multiple depths

TDR Probes Cost = $600

Use of Soil Moisture Measurement Devices Know your soils and crop root zone Contact USDA-NRCS or refer to county soils map to get the field capacity and wilting point Manage the soil moisture in the root zone between the field capacity and 50% of the plan available water (management allowable deficit (MAD); PAW = FC – PWP) example field capacity 12%; wilting point 6%, PAW = 6% point for turning the irrigation on: 9%

Capacitance Probe – Multiple sensors (4, 8, 12, and 20 in) Irrigation Stop - water below root zone

Selection of Devices Factors for selection Size and management of the farm Soil-hydrologic factors How closely you want to fine tune irrigation Cost limitations $200 to 17,00 Personnel available Be aware of the sensor accuracy Especially for sandy soils (FC = 9%; WP = 4%, PAW = 5%) If possible, use multiple depth probes Using factory calibration does not always mean accurate results Most of the available probes are suitable for basic scheduling decisions

Location of Soil Moisture Measurement Factors for selecting locations Soil series map (soil spatial variability) Known wet and dry areas Type of irrigation More the better Multiple soils within the irrigation zone Example: 50 acre zone 49 acre, high WHC; 2 acre, low WHC Soil moisture from 49 acre for managing irrigation 30 acre with high WHC and 20 acre with relatively low WHC Install multiple probes or use the moisture from 20 acre

Location of Soil Moisture Measurement Example Multiple sensor probes (cost and soil dependent) Fruit crops (orchards): Citrus - 10 probes for 1000 acres Row crops: Vegetable - 5 probes for 500 acres Manual/Portable Take measurements from different soil (and crop types) With experience, can do good irrigation scheduling Type of irrigation Install the probe within wetted area of the drip/microsprinkler

Use of Telemetry Wireless transmission of data from field to office Requires less time (travel and personnel) Current data can be available on the internet anywhere, anytime Enviroscan – Wireless logger C-Probe Lease the system Yearly contract