Soil Moisture Measurement for Irrigation Scheduling

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

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

3. 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]

4. 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

5. BREAK / DEMO

6. 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

7. 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

8. 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

9. 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

10. Examples of Types of Devices

11. Capacitance Probe

12. Portable Capacitance Probe Cost = $2000

13. 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

14. Cost = $600 TDR Probes

15. 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%

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

17. 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

18. 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

19. 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

20. 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