Soil Moisture Monitoring in Manitoba · 2020. 12. 14. · Soil Moisture Monitoring in Manitoba Timi...
1
Soil Moisture Monitoring in Manitoba Timi Ojo and Alison Sass Agri-Resource Branch, Manitoba Agriculture and Resource Development. Introduction Soil moisture, though a tiny fraction of the global hydrologic cycle at <0.02%, is important for crop yield and productivity. The availability and mobility of crop nutrients as well as the distribution and rate of activity of soil microbial populations are influenced by the moisture status of the soil. Soil moisture considerations are important for many farm management decisions from seeding to post-harvest operations. Over the last 30 years, there has been a dramatic increase in the development of sensors that monitors soil moisture. In 2018, the global soil moisture sensor market was $129 million USD and is projected to reach $346 million USD by 2025 1 . The spatial heterogeneity of soil and spatio-temporal variability of soil moisture make monitoring soil moisture challenging. 1 https ://www.globenewswire.com/news-release/2019/08/19/1903435/0/en/Global-Soil-Moisture-Sensor-Market-Will-Reach- to-USD-346-Million-By-2025-Zion-Market-Research.html 2 https:// www.gov.mb.ca/asset_library/en/2011flood/flood_review_task_force_report.pdf 3 Ojo, E. R., P. R. Bullock, and J. Fitzmaurice, 2015: Field Performance of Five Soil Moisture Instruments in Heavy Clay Soils. Soil Sci. Soc. Am. J., 79, 20–29, https://doi.org/10.2136/sssaj2014.06.0250 4 https ://www.gov.mb.ca/agriculture/environment/soil-management/manitoba-fall-soil-moisture-survey.html 5 https ://www.grainews.ca/columns/les-henry-finally-soil-moisture-measuring-meets-new-tech/ Soil Moisture Monitoring Drivers Manitoba is one of the few provinces in Canada that monitors soil moisture. Starting in 2016, ML3 Theta-probe sensors were installed at 5, 20, 50 and 100 cm depths at about 107 weather station locations within the agricultural region of the Province (Figure 1). Eleven new weather station installations are planned for 2021 to fill spatial gaps. Increase in weather monitoring and the inclusion of soil moisture sensors were based on the recommendations from the Manitoba 2011 Flood Review Task Force Report 2 . The selection of soil moisture sensor was based on research 3 conducted at the University of Manitoba to determine the pre-calibration stability of several widely used sensors and their adaptability for use in Red River clay. Prior to the installation of sensors, Manitoba Agriculture and Resource Development conducted annual fall soil moisture surveys 4 since 2004 to determine the amount of moisture within the root zone before freeze-up. This provides a good indication of available moisture next spring. Dr. Les Henry famously said “water in the ground is a certainly, rainfall is a probability” 5 . Historically, the Manitoba soil moisture survey involved government employees collecting soil samples usually between the last week of October and the first week of November. Despite the reduced loss to evapotranspiration at that time of the year, rainfall or warm weather after the samples are collected may impact the accurate determination of fall soil moisture. The 2018 fall moisture survey was used as a baseline to compare ground sampling and data from sensors. The results were comparable and digital observations from the sensors were used in 2019 and onward. The soil moisture sensor-monitored data at freeze-up are usually noticeable due to the sharp drop in the soil moisture value once the moisture in the soil turns to ice (Figure 2). The permittivity difference between water (about 80) and ice (about 3) drives the drop in the soil moisture value once a change in state occurs. The sensor voltages are related to the dielectric permittivity of the soil which is then converted to soil moisture. Sensors installed at 5, 20, 50 and 100 cm depths were used to represent soil moisture at 0-10, 11-30, 31-70 and 71-120 cm, respectively. The amount of available moisture (mm) is the water content observed prior to freeze-up which can be made available to plants. The percent of available water holding capacity relates the available moisture at freeze-up to the water holding capacity of the soil which is the maximum amount of moisture that the soil can hold for crop use. The water holding capacity (WHC) of a soil is mainly determined by the soil texture and the amount of organic matter. Clay soils have higher WHC than sandy soils due to larger surface area and higher total porosity. However, the percent of Available Water Holding Capacity (AWHC) standardizes the observations regardless of the soil type. The 2020 fall soil moisture maps were based on data from 102 weather stations across agro-Manitoba. Freeze-up at the 5 cm depth occurred on November 9 at most locations. The southeast corner and many locations in the southwest have less than 150 mm of available moisture but are mostly above 60% AWHC. Many areas in the Interlake region have less than 40% AWHC (Figure 3a & b). Additional maps showing surface and subsurface soil moisture prior to soil freeze-up can be found on the Manitoba Agriculture and Resource Development website: https ://www.gov.mb.ca/agriculture/environment/soil-management/manitoba-fall-soil- moisture-survey.html Fall Soil Moisture Survey References For more information, please contact: Timi Ojo ([email protected]) OR Alison Sass [email protected]) Agricultural Systems Modeller Agricultural Meteorology Specialist Figure 2: Drop in soil volumetric water content (lower chart) in response to the freezing temperature (upper chart) during the fall transition zone at three of the MB Agriculture Weather monitoring sites. Figure 3: Soil moisture maps showing (a) Available water content and (b) Percent of Available Water Holding Capacity. The maps should be used as a regional guide only and not for site-specific inferences. Figure 1: Spatial coverage of MB Agriculture Weather monitoring sites (MB Mesonet).
Soil Moisture Monitoring in Manitoba · 2020. 12. 14. · Soil Moisture Monitoring in Manitoba Timi Ojo and Alison Sass Agri-Resource Branch, Manitoba Agriculture and Resource Development
