Simulating system-wide effects of reducing irrigation withdrawals in the

Apalachicola-Chattahoochee-Flint (ACF) river basin

By: Steve LeitmanJuly 2015

• about 20,000 sq mile basin•¾ basin in Georgia•1/8 in Alabama and Florida•median annual flowabout 16,000 cfs

Introduction

• For the past several decades water users and managers in the ACF basin have been locked in controversy over the use and management of the watershed and this controversy has led to:– the creation and subsequent termination of the first river

basin compact in the U.S. since passage of the major environmental laws in the 1970s circa 2000.

– multiple lawsuits among the three states in the watershed and the federal government ultimately leading to the State of Florida filing a lawsuit in the U.S. Supreme Court against the State of Georgia in 2014.

Introduction

• In this presentation I will investigate whether a reduction in consumptive losses to stream flow in the Flint River portion of the ACF watershed through the large scale introduction of water-saving irrigation devices and practices can have a substantive positive effect on inflows to the Apalachicola River from the watershed above Jim Woodruff Dam and thereby help put an end to this long-term conflict?

Introduction

• A large majority of the irrigated acreage in the ACF basin above Jim Woodruff Dam resides in Georgia. Of this total acreage: – 77% occurs in the Flint basin – 21% in the Spring Creek basin – 2% in the Chattahoochee basin

• about 20,000 sq mile basin•¾ basin in Georgia•1/8 in Alabama and Florida•median annual flowabout 16,000 cfs

Center Pivot irrigation sites in the vicinity of Camilla, Georgia

Introduction

• Between 2007 and 2010 – about 65% of the irrigation water used in Georgia was

from groundwater sources – 22.5% from surface water sources – 6% from well-to-surface ponds – 6.5% from unknown sources

•The source of irrigation water is important because different sources have different effects on the streamflow.

Introduction

• Surface water withdrawals have a one-to-one affect on streamflow.

• Groundwater withdrawals are more complicated and dependent upon:– whether aquifer and stream are linked in terms of hydrology– the properties of the aquifer from which the irrigation well

takes the water– Distance between irrigation well and the stream– stage or elevation of the surface of the stream– In summation, relationship is more like six-to-one.

Introduction

• Agricultural irrigation practices which could be implemented in the ACF basin to reduce irrigation water consumption include:– agricultural best management practices such as sod-based

rotation (SBR) (50 to 75%)– Low pressure drop nozzles (22.5%) and – the use of high residue cover crops, conservation tillage

and soil moisture sensing (15%).

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Methods

• To evaluate the effects of changing agricultural irrigation demands on the ACF system, an existing systems model of the basin developed in the STELLA modeling platform was used.

• Model was run with current operating rules (RIOP) and current volume of consumptive demands.

• Model was run under climate conditions in the watershed from 1939 to 2008.

Methods

• Agricultural irrigation’s effects on streamflow were increased by 25% (increased demands), held constant (current demands), reduced by 25% (moderate decrease), reduced by 50% (large decrease) and reduced by 100% (rain-fed) to represent alternative levels of adoption and implementation of water saving approaches and technologies.

Results

Modeled flow for the Flint River at Bainbridge, Georgia for 2007 - 2008 with varying levels of agricultural irrigation (cubic meters per second)

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Flow

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Results

Modeled flow for the Jim Woodruff outflow to the Apalachicola River for 2007 – 2008 with varying levels of agricultural irrigation (cubic meters per second)

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1/1/2007 3/1/2007 5/1/2007 7/1/2007 9/1/2007 11/1/2007 1/1/2008 3/1/2008 5/1/2008 7/1/2008 9/1/2008 11/1/2008

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• about 20,000 sq mile basin•¾ basin in Georgia•1/8 in Alabama and Florida•median annual flowabout 16,000 cfs

Results

Modeled elevations at Lake Lanier, West Point and W.F. George reservoirs for 2007 and 2008 with varying levels of agricultural irrigation (meters)

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LAKE LANIER RESERVOIR

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W.F. GEORGE RESERVOIR

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Results

Performance metric for floodplain inundation in the Apalachicola River from varying levels of agricultural irrigation

Results

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Performance metrics for sturgeon habitat availability from varying levels of agricultural irrigation

Results

Frequency of years that flow was below important threshold values and median number of consecutive days flow was below important threshold values for mussel habitat in the Apalachicola River from varying levels of agricultural irrigation.

Results

Comparison of the relative distribution of irrigation withdrawal’s effects on streamflow and reservoir elevation for the period of record and in two drought years.

JW OUTFLOW RESERVOIRS JW OUTFLOW RESERVOIRS JW OUTFLOW RESERVOIRSincrease in irrigation 90.4% 9.6% 36.8% 63.2% 67.4% 32.6%moderate decrease in irrigation 63.2% 36.8% 27.5% 72.5% 44.1% 55.9%larger decrease in irrigation 74.1% 25.9% 40.4% 59.6% 64.5% 35.5%no irrigation 90.9% 9.1% 59.3% 40.7% 95.4% 4.6%

period of record 2007 2008

Conclusions

• The results showed that decreasing the volume of irrigation withdrawal can have non-intuitive effects on the ACF basin because of the rules for management of the storage reservoirs.

• Demand savings incurred upstream do not always directly translate to elevated flows downstream.

Conclusions

• In the ACF basin there is not a water supply problem in normal rainfall years, only during drought years.

• Results showed that in years when there is a lesser need for support from the federal storage reservoirs, nearly all of the water savings from decreasing the effects of consumptive withdrawals for agricultural irrigation in the Flint basin would translate into increased flow in the Apalachicola River.

Conclusions

Modeled elevations at Lake Lanier, West Point and W.F. George reservoirs for 2005 and 2006 with varying levels of agricultural irrigation (meters)

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1/1/2005 4/1/2005 7/1/2005 10/1/2005 1/1/2006 4/1/2006 7/1/2006 10/1/2006

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LAKE LANIER RESERVOIR

186.0

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WEST POINT RESERVOIR

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Conclusions

• But in years when there is a larger need for augmentation support from the federal storage reservoirs, such as in drought years, a considerable percentage of the water savings would translate into higher elevations at the federal storage reservoirs

• In evaluating flow metrics used to translate flow changes to environmental effects it was found that changes to flow would occur at a time and rate which could affect federally listed mussel species but not sturgeon spawning or floodplain inundation.

Conclusions

• During drought years, under current reservoir operating rules (RIOP), significant decreases in agricultural irrigation withdrawals results in: 1) supplemental releases not being made from the upstream storage reservoirs in the Chattahoochee basin and consequently, 2) higher elevations especially at Lake Lanier as well as, 3) having some of the increased flow in the Flint basin resulting in increased flow in the Apalachicola River.

Conclusions

• The results of this study show that adopting alternative agricultural practices that reduce irrigation water demands could have substantial effects in the ACF basin.

• Based on ongoing research on agricultural irrigation practices in the ACF basin at both the NFREC and the Stripling Irrigation Park, it seems plausible that irrigation demands could be decreased substantially in the future if alternative practices are implemented at a large scale.

Conclusions

• The extent of this effect is dependent on the level at which these practices are implemented and whether it is a normal year or a drought year.

• The findings from this study suggest it may be beneficial to – pursue means and policies that would implement water

saving agricultural practices, – conduct a systematic review of reservoir operations and

management in order to share increased flow from reduced consumptive extractions in a manner that is understood and acceptable to basin constituents.

Conclusions

• If the intent is to remedy flow related problems in the Apalachicola River and Bay during droughts through reducing agricultural irrigation water use, then the plan for managing the federal storage reservoirs will have to be modified.

• The Corps of Engineers, Mobile District is in the process of revising the Water Control Manual for the basin at the present time.

Conclusions

• The obvious question is: What has to be done to get broader implementation of conservation practices such as sod-based rotation?

• Questions?