The Effect of Soil Hydraulic Properties and Deep Seepage Losses on Drainage Flow using DRAINMOD Debjani Deb 26 th April, 2004

Introduction Drainage is the practice of removing excess water from the land and is used as one of the most important land management tools for improved crop production. The portion of rainfall or applied irrigation water that in excess of the water holding capacity passes through the rooting zone and is subsequently unavailable for crop use is known as Deep Seepage. Deep seepage contributes a portion of base flow in the stream flow.

Background A hydrological assessment using DRAINMOD has already been done on the Animal Science Watershed based on the following assumption:  The impermeable layer of the watershed was at 6 ft deep with no Deep Seepage  The estimation of Soil Types was based on visual assessment from the USDA soil map. Based on the assumptions the previous study had the following limitation:  Since Deep Seepage losses contribute to the base flow and Deep Seepage losses were not taken into consideration, the analysis of the Base Flow in the previous study is not precise.  Estimation of Soil Types were not accurate and hence introduced errors in the analysis

Objective Assess the error encountered in drainage flow due to uncertainty in soil hydraulic properties and deep seepage losses

Model Description DRAINMOD is a computer simulation model, designed for soils with high water table and subsurface drains. Simulates the hydrology of a poorly drained soil and models the field scale effects of drainage on related water management systems.

Governing Water Balance Equations…… The water balance for a time increment ∆t can be written as, ∆Va = D + ET + DS – F The amount of run-off and storage is computed from the water balance at the soil surface P = F + ∆S + RO

Methodology Designate Test Area (Animal Science Watershed, Tippecanoe County). Obtain Data needed for analysis (referred to Rhea Sammon’s thesis’ ABE 2002)

Perform DRAINMOD Simulations Calibrate the model. Discuss the results and perform a sensitivity analysis based on the results

Results  Variation of drainage with Soil Types (Drummer and Toronto 1)  Variation of drainage and seepage with respect to the design parameters (Sensitivity Analysis)

Variation of drainage with Soil Types Hydraulic Conductivities: Drummer: 0.17 cm/hr Toronto 1: cm/hr

Sensitivity Analysis VariablesValues of Input Parameter Drainage (cm) (output) SensitivitySeepage (cm) (output) Sensitivity Min p 1 Max p 2 Min b 1 Max b 2 Min b 1 Max b 2 Drain Depth Drain Spacing Effective Radius of Drain Drainage Co-efficient Initial Depth to Water Table Depth to Restricting Layer Thickness of the Restricting layer Vertical Hydraulic Conductivity of the restricting layer

Discussion The drainage is highly sensitive to the following parameters: Drain Depth Drain Spacing Soil Types Initial Depth to Water Table Hydraulic Conductivity of the Restricting Layer Thickness of the Restricting Layer Depth to the Restricting layer

Discussion Cont…… Whereas Seepage is highly sensitive to the following parameters: Initial Depth to Water Table Hydraulic Conductivity of the Restricting Layer Thickness of the Restricting Layer Depth to the Restricting layer

Conclusion  The sub-surface drainage flow is highly sensitive to the Soil Type and also on the depth, thickness and hydraulic conductivity of the restricting layer.  Higher Hydraulic conductivity leads to reduced drainage

Conclusion Cont…… Decrease in the depth of the Restricting Layer increases drainage Increase in the thickness of the restricting layer increases drainage Decrease in the hydraulic conductivity of the restricting layer increases drainage

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