In many farmed areas, intensive application of agricultural chemicals and withdrawal of ground water
for irrigation have led to water quality and supply issues; however, contributing unsaturated zone processes are not well understood. In the Bogue Phalia basin,
an intensely agricultural area in the Delta region of Mississippi, the fine textured soils often exhibit surface ponding and runoff after irrigation and rainfall as well as
extensive surface cracking during extended dry periods. Fields are typically land-formed to promote surface flow into irrigation ditches and streams that feed into
larger river ecosystems. Downward flow of water below the root zone is considered minimal; regional ground-water models predict only 5 percent or less of
precipitation recharges the heavily-used alluvial aquifer. In this study we assessed transport mechanisms within and below the root zone of a fallow soybean field
by performing a 2-m ring infiltration test with tracers and subsurface instrumentation. Seven months after tracer application, we collected 47 continuous cores for
tracer extraction to define the extent of water movement and quantify preferential flow using a mass balance approach. Vertical water movement was rapid below
the pond (traveling up to 0.21 cm/s) indicating the importance of vertical preferential flow paths in the shallow unsaturated zone. Lateral flow of water at shallow
depths was extensive and spatially non uniform, reaching up to 10 m from the pond within 3 months. Within 2 months, the wetting front reached a textural boundary
at 5 m between the fine-textured soil and sandy alluvium, now a capillary barrier which, prior to extensive irrigation withdrawals, acted to confine the aquifer.
Results indicate that at least 52% of percolation is preferential and that any water breaking through the capillary barrier (potential recharge) likely does so in fingers
which are difficult to detect.
Site Details
- Agricultural: soy beans, cotton, corn, rice, catfish
- Humid Climate: ~130 cm/y precipitation
- Issues: declining water table due to irrigation withdrawals, extensive use of agricultural chemicals
- Prevailing concepts: soils are not very permeable, runoff amounts are great because of low infiltration capacity and land-forming practices, downward flow below root zone is minimal
Research Questions
- Does infiltrated water that is not used by ET go mainly to recharge, lateral flow, both?
- How prevalent is preferential flow?
- How does alteration of shallow soil properties affect infiltration and recharge?
- Soil preparation/tillage, planting
- Crop canopy
- Harvest, stubble
Methods of Investigation
- 67 hours of ponded infiltration- 1.08 m of water over the 2-m ring (shown below, center)
- Calcium bromide and Rhodamine WT tracers
- Subsurface probes to measure water content (TDR) and matric pressure (tensiometers)
- Suction lysimeters to sample for tracers (shown below, right)
- Piezometers to sample for tracers in shallow groundwater and measure water levels
- Continuous cores retrieved after 7 months for bromide analysis and mass balance
Preliminary Results
- The infiltration rate was essentially constant after the first 20 minutes at 2 cm/hr (Kfs was 5.3x10-4 cm/s).
- Probes under the pond responded quite rapidly, as little as 10 minutes for the 30-cm probes and 90 minutes for the 120-cm probes.
- Within 1 month, water was detected at all samplers 0-60 cm depth, and at most samplers (70%) from 60-120 cm depth. There were no detections at 2 m
depth and only one detection at 5 m depth, 2 m away from pond.
- Within 2 months there were detections at every sampler except one, 12 m from the pond. Water reached a textural boundary between the soil and
alluvium at 5 m depth.
- Prior to extensive irrigation pumping, the water table was much shallower and the silty clay layer acted to confine the aquifer.
- 48 continuous cores were extracted to define the wetting front. The figure (right) shows the concentration contours for the upper 1 m of soil.
- Lateral flow of water at shallow depths is extensive and spatially non uniform, reaching up to 10 m from the pond in some directions. Lateral flow
may also be significant at the deeper textural boundary below 4 m.
- Bromide mass balance results indicate the 43% of percolation was preferential based on bormide loss from the unsaturated zone
- Preliminary results indicate that after 7 months any water breaking through the capillary barrier (potential recharge) likely does so in fingers which are
difficult to detect, see conceptual model below. Bromide was detected in one piezometer near the pond, but not in deep core sediment.
Conceptual model of flow through the unsaturated zone
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Last modified: Wed Sep 17 16:01:13 PDT 2003
