Cover crops are frequently used

chapter 7 Water Use and Infiltration Terry L. Prichard Effects on Soil Physical Characteristics, 86 Water Infiltration, 86 Water-Holding Capacity, 87 Cover Crop Water Use, 87 Effects of Cover Crop Management on Water Use, 88 Annual Cover Crops, 88 Perennial Cover Crops, 88 Vegetation Management, 88 Irrigation Systems, 89 Irrigation Frequency, 89 Fertility, 89 Summary, 89 Bibliography, 89 Cover crops are frequently used in the vineyard to improve or preserve soil physical characteristics. Cover crop biomass production and subsequent decomposition can improve soil characteristics, increasing the amount of water that can infiltrate the soil. This improvement, however, depending on the situation, can come at the expense of increased water use. Also, as discussed in chapters 4 and 5, adverse and beneficial effects can result from biological interactions between cover crops and vine crops and from competition for water and nutrients (see Gulick et al. 1994). It is important to choose a cover crop and associated management practices that provide the best array of benefits. This chapter discusses the effects of cover crop use on the soil, the competitive use of water by annual and perennial cover crop species, and various management strategies that help ensure the successful use of cover crops. Effects on Soil Physical Characteristics Water Infiltration Water infiltration rates in most soils are influenced to a large extent by how vulnerable the soil is to crusting. The nature, thickness, and permeability of soil surface crusts vary from one soil type to another depending on the physical, chemical, and biological properties of the soil. Surface crusts constrain water intake rates of soil (Boiffin and Monnier 1986; Helalia, Letey, and Graham 1988; Radcliff et al. 1991), thereby increasing surface runoff and erosion. Surface crusts develop when water is applied to the soil surface either through irrigation or rainfall. Although several mechanisms have been proposed to explain how crusts form (McIntyre 1958; Chen et al. 1980), most surface crusts result from the disruption and slaking of surface aggregates and the eventual reorganization and deposition of suspended particles. Any management practice, including the man- 86

agement of cover crops, that minimizes disruption of surface aggregates can be expected to minimize crusting and its attendant deleterious effects on water intake, helping maintain good infiltration rates. To be effective at minimizing soil crusting, a cover crop must grow densely enough to cover the ground and protect the surface. Total ground cover is the most important factor in achieving good final infiltration rates, according to a 1982 study in New Mexico (Wood, Wood, and Tromble 1987). A comprehensive review of possible management practices for the control of surface crusting was made by Kemper and Miller (1974). These control measures can be grouped into those that ensure adequate cover of the soil surface and those that improve aggregate stability. Cover crops and plant residue mulch provide protection for the soil surface through the interception and reduction of impact energy of falling water drops (Duley 1939; Moldenhauer and Kemper 1969) and through improved aggregation of the surface soil (Barber 1959). A cover crop study in an almond orchard found that using Blando brome, resident vegetation, and strawberry clover as cover crops not only reduced surface soil crusting as measured by soil strength by 38 to 41 percent but also increased steady-state or final infiltration rate by 37 to 41 percent and cumulative water intake by 20 to 101 percent (Folorunso et al. 1992). In the same study, differences in cumulative infiltrated water measured after 120 minutes of irrigation were greatest toward the end of the irrigation season (Prichard et al. 1991). Annual cover crops used as dead mulches, whether standing or mowed, can protect the soil surface, cut soil surface evaporation, and suppress warm-season weeds (Grimes, Goldhamer, and Munk 1990). The reported effect of cover crops on plant growth and yield are mixed. However, if soil water content can be increased and water is needed by the crop, growth and yield tend to increase. A Blando brome cover crop, whether allowed to grow or killed with herbicide, was found to improve plant-water relations for vines under furrow irrigation; however, the practice did not increase plant yield (Gulick et al. 1994). In another study, a killed turf system improved water infiltration, increased aggregate stability, and increased the number of soil macropores; additionally, the yield and growth of peach trees increased due to the improved soil water content (Welker and Glenn 1988). Water-Holding Capacity In order to significantly increase water-holding capacity, pores must be created that can hold water. The use of cover crops supports soil aggregate formation and stabilization, which in time can lead to an increase in water-holding capacity in surface soil (at a depth of 0 to 6 inches [0 to 15 cm]). However, increased water-holding capacity in surface soil can be expected to result in minor increases in root zone water-holding capacity of a deep-rooted crop such as grapevines. A 4-year study was conducted using annual and perennial covers on a sandy loam soil (Prichard et al. 1989). When water-holding capacity of the root zone was measured at the end of the study, no significant differences were found among the treatments, which included a bare soil treatment. Cover Crop Water Use Water infiltration benefits from cover crops must be evaluated in light of the cover crops additional water consumption. Recent studies indicate nearly 300 pounds (136 kg) of water are required to produce 1 pound (0.45 kg) of aboveground cover crop dry matter (Meisinger et al. 1991). One study found that Central California orchard water use was 10 to 30 percent higher under continuous cover cropping than when treated with herbicide (Prichard et al. 1989). The study also found that when Blando brome was grown and used as a mulch throughout the summer, seasonal water use equalled that of bare soil. Gulick et al. (1994) found that the increased infiltrated water due to continuous cover cropping also increased water use by 46 percent versus bare soil. That study also found that water use increased 19 percent over bare soil where a Blando brome cover crop was killed by a contact herbicide in the spring and the residue was maintained throughout the summer months. Cover crops can compete with vines for water and nutrients, reducing vine performance, and they can also suppress root growth near the soil surface in vineyards and orchards (Van Huyssteen and Weber 1980; Moriat 1981; Haynes 1980). Cover crops also tend to reduce grape yields if the cover crops are not rotated every few years, especially in young vineyards under dryland conditions (Stevenson, Neilsen, and Cornelsen 1986; Saayman and Van Huyssteen 1983). Water Use and Infiltration 87

Effects of Cover Crop Management on Water Use Annual Cover Crops Infiltrated winter rainfall serves two important purposes in vineyards: it recharges the root zone with water that the vines ultimately use during the season, and it leaches accumulated salts away from the root zone. The volume of water required for these purpose is variable. For soil recharge, the volume of water required depends on the water-holding capacity of the root zone; for leaching, the water required depends on the level of salinity in the soil and in the irrigation water. In areas of high rainfall, salts are leached effectively and do not accumulate to damaging levels. In areas where rainfall is equal to or less than cover crop water use, all the rainfall would be consumed in the production of cover crop biomass. In order to produce significant quantities of cover crop biomass in areas of low rainfall, irrigation water is required. Even in areas of adequate winter rainfall, increases in cover crop biomass production result from irrigation during early fall. This practice allows the cover crop to become established early and take advantage of warm temperatures, which enhance growth. In general, winter annual cover crops use less water than those that grow during summer months or those that grow year-round. During the winter and early spring, water demand is lower (by virtue of cooler temperatures and shorter day length) and growth occurs during a time when rainfall is likely. A vetch cover grown in Davis, California, from November through early March produced nearly 6,000 pounds per acre (6,720 kg/ha) of aboveground dry matter (Shennan 1992). Soil water contribution between November and March measured less than 0.5 inch (1.2 cm) in a below-normal rainfall year (normal annual rainfall averages 18 inches [46 cm]). The water required to produce the 6,000 pounds per acre of dry matter was mostly supplied by winter rainfall. Using the relationship of 300 pounds (136 kg) water to produce 1 pound (0.45 kg) of dry matter, a 6,000-pound (6,720-kg) dry matter cover crop would use approximately 8 inches (20.5 cm) or 217,000 gallons per acre (2,030 kl/ha) of water. If water were available from rainfall or irrigation and the crop were growing in the higher water-demand months of March and April, water consumption would have been substantially higher. Water use by the cover crop may significantly decrease the amount of water stored in the root zone. Increased irrigation may be necessary to meet the vine water needs and salt leaching requirement. In areas where irrigation water is unavailable or in limited supply, this may be of great concern. As an example, the cover crop biomass that was produced in the Davis study would have resulted in no winter rainfall stored for subsequent vine use in vineyards during the growing season. Perennial Cover Crops Perennial cover crops, which live year-round, offer the advantage of not having to be reseeded each year. However, the stand must be maintained throughout the season, and at least small amounts of water are required to maintain the stand from late spring through early fall. Resident vegetation, usually consisting of winter annuals followed by summer annuals, also fits in this category since it can grow and use water for the entire season. Perennial grasses that are less active during the summer offer some hope of less competitive water use. In the North Coast under cooler climate regimes, many perennial grasses can survive without additional irrigation. As previously mentioned, continuously grown cover crops can use 19 to 46 percent more water than a bare vineyard floor. Vegetation Management Vegetation management techniques can also affect the ultimate volume of water that is consumed and the biomass produced. Disking of cover crops at bud break of grapevines on flat land or after the erosion hazard period on sloped vineyards can cut water use by the winter cover crop. Sublethal postemergence herbicide applications (chemical mowing) can substantially reduce water use and biomass production of the cover crop. In an almond cover crop experiment, a chemical mowing program allowed vegetation to grow from spring through summer at a reduced rate (Prichard 1994). Near harvest, the vegetation was completely removed. Using this management technique, the water use was no more than that of the Blando brome or bare soil. The volume of water used and the biomass produced was found to be directly related to the rate and frequency of herbicide application. 88 Chapter 7

Irrigation Systems In areas of adequate and early rainfall, cool-season cover crops can be maintained with all types or irrigation systems. Full-coverage systems (such as flooding between borders, furrow irrigation, and the use of sprinklers) provide the best opportunity for warm-season cover crop growth. Some microirrigation systems, such as aboveground and underground drip, do not provide moisture for cover crop growth in vine centers due to their point-source emission. Depending on the area of coverage, microsprinklers can provide some water for the maintenance of cover crops. Irrigation Frequency Most cover crops are more shallow rooted than grapevines. After an irrigation, they compete directly with the vines for available moisture. As the moisture is depleted in the shallow area of the root zone, the vines acquire moisture at lower depths, which are generally inaccessible to the cover crop. Frequent irrigation can allow the cover crop to compete with the vines, leading to increased biomass production at the expense of increased water use. Vineyard irrigation volume and frequency varies with region but can range from none to many irrigations per season. In the Lodi area, for example, sufficient wine grape water use without the presence of a cover crop averages about 18.5 inches (47 cm) per year. Soils of this area typically store 8 to 12 inches (20.5 to 30.5 cm) of rainfall in the root zone for subsequent vine use. The remaining 6.5 to 10.5 inches (16.5 to 26.5 cm) of the requirement is applied in 1 to 2 surface or 2 to 3 sprinkler irrigations. Generally, this is not frequent enough to promote cover crop growth or stand survival during the summer months. Maintaining a cover crop in this region is possible until early June in a normal rainfall year with a single May irrigation provided for the cover and vine. In the cooler climate of the North Coast, soils may store adequate water to supply the vine water requirement without irrigation, making it difficult if not impossible to maintain a summer cover crop. Fertility Wine grape vineyards are fertilized with relatively low amounts of nitrogen. Typical broadcast applications are from 30 to 50 pounds per acre (33.6 to 56 kg/ha) of nitrogen per year. When injected into microirrigation systems, the rates are reduced to 10 to 20 pounds per acre (11.2 to 22.4 kg/ha) per year. Broadcast fertilizer applications are usually made in the spring, leaving little nitrogen available at a time when the annual cover crop germinates or the perennial crop begins to grow after fall rains. To increase cover crop biomass production, it is common to fertilize in the fall while temperatures are still warm. Our research in wine grape vineyards has shown a 1,000 percent biomass production increase by applying 30 pounds per acre (33.6 kg/ ha) of nitrogen at germination of resident vegetation (October). Biomass collection occurred on March 31. Additionally, if 2 inches (5 cm) of sprinkled water was applied in early October prior to rainfall, an additional 100 percent increase in biomass was produced, totaling 5,000 pounds per acre (5,600 kg/ ha) for the fertilizer plus water treatment. Summary Cover crops grown as companions with grapevines can accomplish various goals, including the production of biomass to protect the soil from formation of surface crusts. Upon organic matter decomposition, the products stabilize soil aggregates, improving water infiltration rates. Cover crops can, however, compete with the vines for water and nutrients. Depending on the situation, this may be an advantage or disadvantage. It is a disadvantage when water supplies are limited, leading to reduced grape production, or when the cover crop uses winter rainfall that is needed for salt leaching. It is an advantage if the grower desires to bring about early-season water deficits by limiting the volume of rainwater stored in the root zone. Depending on the goal, cover crops can provide a wide array of benefits if careful selection of species and management practices are used. Bibliography Barber, S. A. 1959. The influence of alfalfa, bromegrass and corn on soil aggregation and crop yield. Proceedings of the Soil Science Society of America 23:258 259. Boiffin, J., and G. Monnier. 1986. Infiltration rate as affected by soil surface crusting caused by rainfall. In Callebaut et al. eds., Assessment of soil surface sealing and crusting. Ghent, Belgium: Flanders Research Centre for Soil Erosion and Soil Conservation. Chen, Y., J. Tarchitzky, J. Brouwer, J. Morin, and J. Banin. 1980. Scanning electron microscope observations on soil crusts and their formation. Soil Science 130:49 55. Duley, F. L. 1939. Surface factors affecting the rate of intake of water by soils. Proceedings of the Soil Science Society of America 4:60 64. Water Use and Infiltration 89

Folorunso, O. A., D. E. Rolston, T. Prichard, and D. T. Louie. 1992. Soil surface strength and infiltration rate as affected by winter cover crops. Soil Technology 5:189 197. Grimes, D. W., D. A. Goldhamer, and D. Munk. 1990. Cover crop management and modified drip irrigation for improved water infiltration. Davis: University of California Kearney Foundation of Soil Science Annual Report. 128 135. Gulick, S. H., D. W. Grimes, D. S. Munk, and D. A. Goldhamer. 1994. Cover-crop-enhanced water infiltration of a slowly permeable fine sandy loam. Journal of the Soil Science Society of America 58(5)1539 1546. Haynes, R. J. 1980. Influence of soil management practice on the orchard agro-ecosystem. Agro-Ecosystems 6:3 32. Helalia, A. M., J. Letey, and R. C. Graham. 1988. Crust formation and clay migration effects on infiltration rate. Journal of the Soil Science Society of America 52:251 255. Kemper, W. D., and D. E. Miller. 1974. Management of crusting soils Some practical possibilities. In Cary and Evans, eds., Soil crusts. Tucson: University of Arizona Agricultural Experiment Station, Technical Bulletin 214. McIntyre, D. S. 1958. Soil splash and the formation of surface crusts by raindrop impact. Soil Science 85:261 266. Meisinger, J. J., W. L. Hargrove, R. L. Mikkelsen, J. R. Williams, and V. W. Benson. 1991. Effects of cover crops on groundwater quality. In W. L. Hargrove, ed., Cover crops for clean water. Jackson. TN: Soil and Water Conservation Society. Moldenhauer, W. C., and W. D. Kemper. 1969. Interdependence of water from energy and clod size on infiltration and clod stability. Proceedings of the Soil Science Society of America 33:297 301. Moriat, R. 1981. Effect of different cultivation practices on root system of vine and properties of soil. Journal Agronomie (Paris) 1:887 895. Prichard, T. L., R. A. Dahlgren, W. K. Asai, and D. E. Rolston. 1991. The tendency of field soils to form surface crusts and their effects on water infiltration characteristics. Davis: University of California Kearney Foundation of Soil Science Annual Report. 66 75. Prichard, T. L., W. M. Sills, W. K. Asai, L. C. Hendricks, and C. L. Elmore. 1989. Orchard water use and soil characteristics. California Agriculture 43(4):23 25. Radcliff, D. E., L. T. West, R. K. Hubbard, and L. E. Asmussen. 1991. Surface sealing in coastal plains loamy-sands. Soil Science Society of America Journal 55:223 227. Saayman, D., and L. Van Huyssteen. 1983. Preliminary studies on the effect of a permanent cover crop and root pruning on an irrigated Colombar vineyard. South African Journal of Enology and Viticulture 4:7 12. Shennan, C. 1992. Cover crops, nitrogen cycling, and soil properties in semi-irrigated vegetable production systems. Horticultural Science 27(7):749 754. Stevenson, D. S., G. H. Neilsen, and A. Cornelsen. 1986. The effect of woven plastic mulch, herbicides, grass sod, and nitrogen on Foch grapes under irrigation. HortScience 21:439 441. Van Huyssteen, L., and H. W. Weber. 1980. The effect of selected minimum and conventional tillage practices in vineyard cultivation on vine performance. South African Journal of Enology and Viticulture 1:77 83. Welker, W. V. Jr., and D. M. Glenn. 1988. Growth responses of young peach trees and changes in such characteristics with sod and conventional planting systems. Journal of American Society of Horticultural Science 113:652 656. Wood, J. C., M. K. Wood, and J. M. Tromble. 1987. Important factors influencing water infiltration and sediment on arid lands in New Mexico. Journal of Arid Environments 12:111 118. Prichard, T. L. 1994. Cover crop management influences biomass production and nitrogen weed science extraction. Proceedings of the 46th Annual California Weed Science Society. Jan. 17, 1994. 90 Chapter 7