The impacts of irrigation to the hydrological processes in the unsaturated zone beneath almond grove orchards

Author: Sangita Dandekhya

Dandekhya, Sangita, 2019 The impacts of irrigation to the hydrological processes in the unsaturated zone beneath almond grove orchards, Flinders University, College of Science and Engineering

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This study is a part of an ongoing research project that is investigating the hydrological processes within the unsaturated zone of irrigated almond orchards. One of the key processes to understand is root zone drainage (RZD) which subsequently affects the total recharge to the regional groundwater aquifer system and potential salinisation of the Murray River. The field research was conducted at an irrigated almond orchard, located near Boundary Bend, Victoria, Australia. Two near-surface geophysical methods, electrical resistivity tomography (ERT) and terrain conductivity frequency domain electromagnetic induction (FDEM) surveys were used to understand the subsurface geology of the study area. Water levels in both the perched aquifer and regional aquifer systems were monitored to examine the hydraulic behaviour and perching of the shallow water table. The monitoring of the shallow drainage systems enabled estimation of the volume of water that was being captured from the perched aquifer. The comparison of the 2D resistivity profiles from the ERT surveys with stratigraphic and lithological logs confirmed the occurrence, depth and thickness of alluvial material (13 m thick) and Blanchetown Clay Formation (10 m thick) below the land surface. The terrain conductivity surveys also confirmed the continuity of the Alluvial Formation and Blanchetown Clay throughout the study area. This clay layer acts as a semi-confining layer and thereby intercepts the downward transport of RZD, resulting in a perched aquifer condition. Water level monitoring showed a decreasing hydraulic gradient of both the regional aquifer system and the shallow perched aquifer towards the Murray River. The watertable in the perched aquifer is approximately 2m above the regional water table. The monitoring of inflow in nine shallow drainage pits over four months from late April to early September 2019 was used to calculate the total volume of water (885 m3) that was perched over the clayey formation in this time period. Averaging this volume over the year, the rate of RZD is calculated to be around 0.80 mm/yr. However, the rate of RZD is far lower than the irrigation rate (1000 mm/yr), which suggests high efficiency use of irrigation water by almond trees or some RZD directly recharging the aquifer that is bypassing the shallow drainage pits. In summary, the RZD is contributing to the formation of shallow perched aquifer conditions over the alluvial clay layer in the study area, which limits the total recharge to the regional water table. Nevertheless, further study is deemed necessary to quantify the total recharge under irrigated agriculture and its implications to salinisation of the nearby Murray River.

Keywords: irrigation, root zone drainage, clay layer, perched aquifer, groundwater recharge, geophysics, resistivity, frequency domain electromagnetic induction, salinity

Subject: Hydrology thesis

Thesis type: Masters
Completed: 2019
School: College of Science and Engineering
Supervisor: Professor Peter Cook