Measuring Spatial and Temporal Dynamics of Drip Irrigation Applications to Nursery Tree Crops

One of the key issues in sensing soil moisture is accurate sensor placement, since it is critical for precision irrigation applications to ensure continuous growth, conserve water and limit nutrient leaching.  Precise sensor placement also optimizes data quality (reducing data overload), thereby minimizing the cost of implementing sensor networks.  We are investigating this issue in diverse ornamental production environments, in soil (tree farms) and soilless substrates (container production).  Our primary challenge comes from sensing small-volume drip irrigation applications to trees with large root volumes.  A study was initiated in spring 2009 to monitor drip irrigation applications on newly transplanted Acer rubrum ‘Red Sunset’ trees, in a Duffield-Ryder Silt loam soil.  The soil volumetric water content of three replicate trees was sensed every 15 min with 18 Ech₂0-10HS (Decagon Devices) sensors in three dimensions, from April through Nov, 2009.  Sensors were inserted at 15, 30 and 45 cm depths central to the tree, and at 30 cm either side of the tree, both horizontal (along the row) and lateral (in the row),  at a distance of 30 cm from the trunk.  The primary first-year objective of the study was to ascertain the movement of irrigation water applied by two drip emitters (each applying 36 mL / min) at 30 cm either side of the trunk in the row.  During a dry period between significant rainfall events (8/28/09 and 10/14/09), a total of 320 L water was applied to each tree in 12 irrigation events.  Each irrigation was split into 2-4 cyclic events (of equal volume) to see if irrigation duration had a significant effect on water penetration and lateral movement.  Four short irrigation events showed significant lateral movement of water into the rootball, compared to two longer irrigation events, at both 15cm and 30 cm depths.  The study will continue during 2010, to ascertain whether these observations will change with increasing root density in the soil volume.