Valley Habitats: Number Three

ENHANCING AGRICULTURAL FIELDS FOR WATERFOWL

Historically, California wetlands have accommodated one of the largestconcentrations of wintering waterfowl in the world. An estimated 5 millionacres of wetlands were found in California during the mid-1800s. Wetlands werefound statewide, from the Modoc Plateau and Klamath Basin to the Imperial andCoachella Valleys. However, the majority of these wetlands occurred in theCentral Valley. The Central Valley averages 40 miles wide and is 400 mileslong. It is bounded by the Klamath Mountains to the north, Tehachapi Mountainsto the south, the Sierra Nevada to the east, and the Coast Range to the west.

The Central Valley is one of the most important areas in North America forwintering waterfowl, but no other wintering area has experienced as great awetland loss. To date, about 95% of California's wetlands have been modifiedor destroyed. Two-thirds of the estimated 383,000 acres of wetlands left inthe Central Valley are privately owned. The remaining one-third is owned bystate and federal agencies and managed as wildlife areas or national wildliferefuges.

The wetlands and agricultural lands of California's Central Valley supportabout 20% of all wintering waterfowl in North America and more than 50% ofseveral species (e.g., northern pintail, white-fronted goose, tule goose andcackling and Aleutian Canada geese). Because of the limited natural wetlandarea, the large numbers of waterfowl wintering in California could not besupported without small grain agriculture. Knowledge of waterfowl ecology isextremely helpful for developing supplemental habitat for wintering waterbirdson agricultural fields in the Central Valley. These management practices notonly benefit waterfowl but many other species of migrating and residentwildlife as well.

The density of wintering waterfowl on such limited wetland acres is greater inCalifornia's Central Valley than anywhere else in North America. With as manyas 10 to 12 million migrating waterfowl, the need for wintering habitat isevident. Although this paper presents guidelines for enhancing small grainfields for wintering waterfowl habitat, these practices also have beensuccessful with non-traditionally enhanced crops such as tomatoes andasparagus.

FOOD VALUE OF FLOODED GRAIN FIELDS

Waterfowl arriving in the Central Valley require a diet rich in carbohydratesto replenish fat reserves lost during fall migration. Agricultural crops areeaten by many species of waterfowl because they are widespread, easilyaccessible, and provide high levels of carbohydrates. Crops like corn, wheat,rice, barley, oats, peas, sorghum, rye, millet, soybeans, and buckwheatcommonly are managed as waterfowl foods.

Click here for Figure 1.

Most species of waterfowl must meet similar nutritional requirements duringeach annual event. For example, all species have increased proteinrequirements during molt and egg laying. Female dabbling ducks (e.g., mallard,pintail, and gadwall) undergo a molt during late winter, the prebasic molt.The increased protein requirements of dabbling ducks to fulfil importantlifecycle needs are met primarily from invertebrates. Agricultural fieldsflooded through late winter may also provide invertebrate food resources formolting and prelaying hens.

WATER LEVEL MANIPULATION AND TIMING

Water level manipulation is one of the most important aspects of managing anagricultural impoundment for wildlife use. At least three requirements areessential to manipulate adequately the water level of the impoundment or field.Most importantly, a dependable water supply must be available. Secondly, thetopography of the impoundment must have an elevation gradient that will allowfor complete coverage at the desired water depth. Finally, the proper type ofwater control structures must be placed to allow effective flooding, waterlevel control, and complete drainage of the impoundment.

Wildlife use of flooded agricultural fields is greatly affected by timing,depth, and duration of flooding and drawdown. There are three very importantaspects to timing of flooding. First, timing of flooding, which is greatlydependant on cropping schedules, should coincide with the fall arrival ofmigrating birds. Ideally, flooding should begin just before the expectedarrival of the first migrants. In the Central Valley, these are generallynorthern pintails, which arrive in the late summer and early fall. At thistime, about 5% of the total area expected to be flooded should be flooded.

The next important aspect of timing is the sequence in which fields areflooded. As the number of migrating birds increases, so should the area offlooded land. Generally, the area flooded should be increased to about 85% atthe peak of migration (usually from mid-December to early January). Theremaining 15% should be flooded after peak migration (Fig.1). This sequentialflooding ensures that newly enhanced fields are available for migrating birdsthroughout the wintering period. This is important because once small grainsare flooded, they begin to loose the carbohydrates that are important towaterfowl. For instance, after flooding for 90 days, rice will lose 19% of thecarbohydrates and corn will lose 50%.

Another integral aspect to the timing of flooding is the rate at which theindividual impoundment is flooded. The rate of flooding should be gradual,over 3 - 5 weeks. This allows a front of water to slowly inundate new ground.Gradual flooding increases the time and area for wildlife feeding at thewaterline by forcing the band of intensive foraging across the width of thefield.

The most important factor involved in flooding impoundments for waterfowl iswater depth. Once the impoundment is flooded, average water depth throughoutthe unit should be about 4 inches. About 90% of the waterbirds will use waterthat is between 2 and 8 inches deep (Fig. 2).

All aspects of the timing of flooding apply to drawdowns as well. After peakmigration occurs (February through April), drawdowns should begin. Drawdowntiming also is dependant on cropping schedules. Ideally, spring drawdownsshould occur in a sequential manner. Sequential drawdown of impoundments willprovide a variety of foraging opportunities for various waterbirds, includingexposed mudflats for shorebirds.

Click here for Figure 2.

Rate of drawdown is also important. To maximize wildlife use of animpoundment, a long drawdown (3-5 weeks) is best for waterbirds. The gradualdraining of a unit will concentrate food items, like invertebrates, in smallerpools. However, in areas with brackish or saline water, a slow drawdown couldincrease soil salinity. In these situations, faster drawdowns (1-3 days) maybe needed.

Most small grain fields are suited to water manipulation and can be managed toproduce large amounts of waterfowl foods. However, winter wheat can beuniquely managed for waterfowl. When practical, winter wheat can be floodirrigated immediately after harvest with up to 3 inches of water. The watershould be left for 2-3 days then allowed to recede. This activity will allownative annuals, such as smartweed and watergrass, to germinate. If the fieldis irrigated again 2 weeks later, the seed production will be maximized. Thiswill provide a greater quantity and diversity of waterfowl foods availableduring the unit's fall flood-up.

WATER CONTROL

Permanent Levees

Permanent levees are an integral component of farming systems and developedwetlands because they permit control of water levels and dictate maximum waterdepth (Valley Habitats, Number 4). The primary goal of levee placement is tomaximize flooded habitat at depths that encourage foraging by waterbirds.Permanent levees also are used to form header ditches for water movement.

An understanding of the soil texture on the site is required to ensure longtermintegrity of levees. Because soils have different chemical and physicalproperties, obtaining suitable material for levee construction is essential.For example, highly organic soils, because of their potential to shrink andswell, are poorly suited for permanent levees. Coarse, sandy soils also aresuited poorly for levee material because they erode readily or fail to holdwater. Because of their high compactibility and low potential to shrink orswell, clays or silt clay loams are generally the best soils for leveematerial. Local Natural Resource Conservation Service (formerly SoilConservation Service) offices can provide assistance with recommendedengineering specifications for levee construction on specific soil types.

Levees should be large enough to support equipment for maintenance andvegetation control (Fig. 3). Construction of large levees with flat slopes maydeter damage from burrowing mammals such as muskrat, beaver, and groundsquirrel. Minimum side slopes of 4:1 or 5:1 with 8-12 foot crowns (Fig. 3a-c)generally suffice for permanent levees. Further, soils for permanent leveesmust be compacted to remain stable.

Click here for Figure 3.

The height and selected width of levees also may depend on the size of theimpoundment and the expected depth of flooding. Large impoundments orimpoundments with the potential for deep flooding are subject to severe waveaction and erosion. Because of variability of soils and risk of wave damage,an engineer's advice should be sought for levee design. As a generalguideline, levee height should be at least 1.0 to 1.5 feet above the maximumplanned flooding depth. Where unplanned flooding occurs regularly, as alongrivers, low levees, that submerge quickly and uniformly, tend to be damagedless than larger levees.

Temporary Levees

Improving water manipulation capabilities on seasonally flooded impoundmentscan often be achieved through the construction of temporary levees, oftenreferred to as rice dikes (Fig. 3d). Levees of seasonally flooded agriculturalimpoundments (4-18 inches water depth) should be at least 3 feet high.Completed rice dike dimensions vary by soil type and construction implementsused, but those constructed with a rice dike plow usually create levees with a2:1 slope, a base width of about 8 feet and a height of 2 feet. However,other implements can be used for constructing temporary levees. Terrace plows,bulldozers, and motor graders can be used to develop levees with more gradualside slopes and greater heights, but construction may be more costly. Ricedikes are particularly susceptible to erosion from wave action because theyhave steep side slopes and are not compacted. As a result, rice dikes usuallylast less than 2 years.

In unleveled fields, rice dikes are most effective if constructed alongelevational contours. Constructing temporary levees along contours allowsmaximum water level control across the flooded area and provides optimal watercontrol for wildlife enhancement.

When impoundments are flooded by pumping water, or whenever possible, higherelevation units should be flooded first. Then, as wildlife requirementsincrease, additional water can be added and staged down sequentially throughthe contour impoundments.

Depending on the area and implement used, establishing borrow areas inside thelevees may be advantageous. In impoundments without much topographicalvariation, inside levee borrow areas can provide deeper water for foragingdepth diversity. This will provide foraging habitats for other waterbirds(Fig. 2). However, an elevated access should be established across any borrowarea inside a levee to allow for equipment access into the unit.

Water Control Structures

One of the most important considerations for water level manipulation is thetype and placement of water control structures. Effective management requiresthe ability to precisely control the water depth. Structures to regulatedewatering should be placed at low enough elevations to allow complete drainingof the impoundment and borrow ditches. Structures should also be of propersize to allow rapid drainage. One of the most economical and effectivestructures are stoplog types (Fig. 4). Stoplog structures are effectivebecause desired changes in water depth can be achieved with varying stoplogsizes and water depths can be maintained with minimum monitoring. Screw gateswork well for water delivery, but should not be used as outlet structuresbecause they require constant monitoring during drawdowns. Size and topographyof the impoundment will dictate the appropriate number and size of watercontrol structures.

Click here for Figure 4.

With the appropriate water control structures in place, quick and completedrainage is attainable. However, in impoundments where topographic variationmakes draining isolated areas difficult, lateral ditches will be helpful.Lateral ditches can be cut from low lying areas to borrow ditches, or directlyto drainage ditches, using a ditch plow or similar implement. In general,lateral ditches can be placed as close together as needed to facilitateadequate drainage. In addition, lateral ditches greatly help reduce soilsalinity.

BENEFITS TO THE FARMING OPERATION

Agricultural operations that incorporate wildlife enhancement can providewildlife observation and hunting opportunities, while substantially benefittingthe farming operation. Post-harvest flooding is the preferred method forcontrolling many undesirable weeds. For example, in the Sacramento-San JoaquinDelta, early post-harvest flooding effectively reduces Johnson grass. Winterwaterbird feeding activity in these impoundments also may contribute to thereduction of the weed seed bank the following year.

In many areas of the Central Valley, wind erosion and subsidence are causes forconcern. Where highly organic soils are present, post-harvest flooding mayretard soil oxidation, a major cause of soil subsidence. Minimal tillagefurther assists with reducing soil loss from erosion.

Post-harvest flooding for wildlife also can be used to control soil salinity.Circulating water through the impoundments may reduce salinity problems ifmanaged in a gradual, sequential flooding regime.

On rice fields, post-harvest flooding followed by rolling has become a widelypracticed method for decomposing rice straw (Valley Habitats, Number 1). It isrelatively low cost and generally not dependent on weather conditions.Although continuous flooding may not be required for rice straw decomposition,open water can attract foraging waterfowl, whose feeding activities andmovement may increase the rate of straw decomposition.

eds. Management of Nongame Wildlife in the Midwest: a Developing Art.North-Cent. Sect., The Wildl. Soc., Grand Rapids, Mich.

Fredrickson, L.H. and F.A. Reid. 1988. Invertebrate response to wetlandmanagement. U.S. Fish and Wildlife Leaflet 13.3.1. 5pp.

Fredrickson, L.H. and F.A. Reid. 1988. Nutritional values of waterfowl foods.U.S. Fish and Wildlife Leaflet 13.1.1. 6pp.

Fredrickson, L.H. and F.A. Reid. 1990. Impacts on hydrologic alteration onmanagement of freshwater wetlands. Pp 71-90 in: J.M. Sweeny, ed. Management ofDynamic Ecosystems. North Cent. Sect., The Wildl. Soc., West Lafayette, Ind.

Fredrickson, L.H. and T.S. Taylor. 1982. Management of seasonally floodedimpoundments for wildlife. U.S. Fish and Wildlife Service Resource Publication148. 29pp.

Gilmer, D.S., M.R. Miller, R.D. Bauer and J.R. LeDonne. 1982. California'sCentral Valley wintering waterfowl: concerns and challenges. Trans. North Am.Wildl. and Nat. Resour. Conf. 47:441-452.

Heitmeyer M.E., D.P. Connelly, and R.L. Pederson. 1989. The Central,Imperial, and Coachella valleys of California. pp 475-505 in: L.M. Smith,R.L. Pederson, and R.M. Kamiski eds.

Habitat Management for Migrating and Wintering Waterfowl in North America.Texas Tech. University Press.

Kelly, J.R. Jr., M.K. Laubhan, F.A. Reid, J.S. Wortham, and L.H. Fredrickson.1993. Options for water-level control in developed wetlands. U.S. Fish andWildlife Leaflet 13.4.8. 7pp.

Reid, F.A. 1993. Managing wetlands for waterbirds. Trans. N. Am. Wildl. andNatur. Resour. Conf. 58:345-350.

Ringleman, J.K. 1990. Managing agricultural foods for waterfowl. U.S. Fishand Wildlife Leaflet 13.4.3. 4pp.

Warner, R.E. and S.J. Brady. 1994. Managing farmlands for wildlife. Pp648-662 in: T.A. Bookhout, ed. Research and Management Techniques for Wildlifeand Habitats. Fifth ed. The Wildlife Society, Bethesda Md.

Fredrickson, L.H. 1991. Strategies for water level manipulations inmoist-soil systems. U.S. Fish and Wildlife Leaflet 13.4.6. 8pp.

Fredrickson, L.H. and F.A. Reid. 1986. Wetland and riparian habitats: anongame management overview. Pp 59-96 in: J.B. Hale, L.B. Best and R.L.Clawson, eds. Management of Nongame Wildlife in the Midwest: a Developing Art.North-Cent. Sect., The Wildl. Soc., Grand Rapids, Mich.