RESEARCH ARTICLE ▼ Converting oak woodland or savanna to vineyards may stress groundwater supply in summer by Mark Grismer and Caitlin Asato Water resources are important to land- use planning, especially in regions where YinYang/iStockphoto converting native oak woodlands or savannas to wine grape vineyards may affect the amount of water available for restoring salmon runs. Research has shown that woodland conversion to grasslands (for possible rangeland graz- ing) leads to greater and more sustained stream flow and groundwater recharge; however, little information is available about woodland conversion to vine- yards. To inform resource managers and planners, we developed a water balance model for soil and applied it to vine- yards, native oak woodlands and annual grasslands to evaluate their relative use In areas where oak woodlands are converted to vineyards, the recharge of groundwater to streams may change. The authors developed a water balance model for soil, which they applied to vineyards, of groundwater. We applied the model oaks and grasslands. Water use, groundwater recharge to Sonoma County, using climate data savanna conversions have been raised in from 1999 to 2011, and determined that terms of losses in landscape ecological Hydrologists have long been interested oak tree canopy coverage of 40% to 60% diversity (Heaton and Merenlender 2000), in the impacts on soil water conditions results in annual groundwater extrac- adverse impacts on soils (Jackson et al. and groundwater recharge when convert- 1990), soil erosion, water quality (Hinck- ing native grasslands or woodland to tion equivalent to that of an established ley and Matson 2011) and water quantity. agriculture in the world’s semiarid and irrigated vineyard. However, vineyard One community concerned about vine- arid regions. Hydrologic analyses con- groundwater use far exceeded that of yard expansions referred to them as the ducted decades ago at the UC Hopland “tentacles of the wine-grape octopus” Research and Extension Center in coastal oak woodlands in late summer to early (Parrish 2011), and local scientists in So- Mendocino County considered land con- fall, which could further stress already noma County have raised concerns about version in terms of watershed water yields affected groundwater resources. We also biodiversity (Community Foundation (see page 145); Burgy and Adams (1977) evaluated the prediction sensitivity of 2009). These concerns are amplified by cli- characterized the focus at that time: mate change, which increases plant water the model to key parameters associated demands, possibly resulting in decreased Quantitative studies of the hy- with rain levels, soil water–holding ca- groundwater availability. And yet, wine drologic responses of watersheds pacity and irrigation management. grape (and associated wine) production where dense vegetative cover has is the leading agricultural commodity in been replaced with range and for- California in terms of net dollar value. age grasses have consistently shown ine grape production in Califor- In Sonoma County, wine grape pro- increases up to 50% or more (equiva- Wnia coastal counties has increased duction increased from less than 50,000 lent to 3- to 5 acre-inches per acre) in steadily during the past few decades, acres in 1998 (Merenlender 2000) to a peak annual runoff over long periods of often resulting in the conversion of native of 70,000 acres in 2002 and has leveled off measurement. These runoff studies oak woodlands or savannas into vine- at around 60,000 acres during the past cover the variety of conditions found yards. Generally, oak woodlands have decade (fig. 1). Concerns about the impacts tree canopy covers greater than about of vineyard water use on salmon runs are 40% by area, while oak savannas are pre- focused here (Lohse et al. 2008) because Online: http://californiaagriculture.ucanr.edu/ dominantly grasslands interspersed with the county includes a large part of the landingpage.cfm?article=ca.v066n04p144&fulltext=yes oaks. Concerns about oak woodland and Russian River watershed. DOI: 10.3733/ca.v066n04p144 144 CALIFORNIA AGRICULTURE • VOLUME 66, NUMBER 4 in Northern and Central California. 70 About half of the yield increase 65 occurs in the latter portion of the season, giving usable flow in dry pe- 60 riods. The balance of the increase is 55 produced as increased outflow dur- 50 ing the post-storm periods. 45 After oak woodlands were converted 40 to annual grassland, the researchers 35 found increased storm runoff volumes and the establishment of perennial 30 summer stream flows. Base flows had Grape acreage (1,000s) 25 increased and overall groundwater demand dropped when oaks were re- 20 moved. Currently, watershed manage- 15 ment encompasses a broader perspective, White grape 10 Red grape considering the land conversion im- Total wine grape pacts on ecological diversity, soils and 5 water quality. 0 Similar observations have been made 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 of increased groundwater recharge fol- Fig. 1. Annual wine grape production in Sonoma County, CA. Source: County annual agricultural lowing the conversion of native land crop reports. use (rangeland) to agriculture in the U.S. Southwest, Central Plains and High Plains (Gurdak et al. 2007; McMahon et al. Glossary 2006; Scanlon et al. 2005), and the Murray- Darling basin of Australia following con- Crop coefficient c(K ): Ratio of actual below; such recharge typically occurs version of native woodland (mallees) to crop water use to reference ETo; used to later in the rainy (winter) season. dryland farming and pasture (Thorburn determine irrigation water demand in Runoff: That part of the precipitation et al. 1991). vineyards and other crops. that appears in surface streams; may Scanlon et al. (2005) noted that ground- Effective rainfall: Fraction of total come either from the surface, or from water recharge under rangeland use is rain that infiltrates the soil after losses shallow groundwater (the latter is usu- typically nonexistent in arid regions and to leaf interception, surface runoff, de- ally referred to as interflow). quite small in some semiarid regions pression storage and evaporation. Soil water balance: Relatively simple (~ 1 millimeter per year); it increases to Evapotranspiration, plant: Sum of model that accounts for daily changes roughly tens of millimeters annually in soil water and canopy (leaf) evapora- in water storage in the root zone, as- dryland agriculture and hundreds of mil- tion, plus plant transpiration, a pro- sociated with such processes as root limeters in irrigated agriculture. The an- cess in which water moves through a water extraction (ETc), infiltration by nual recharge rate in semiarid regions is plant or tree and is subsequently lost rain or irrigation, and seepage losses to likely rainfall dependent; for example, in through stomata in the leaves. groundwater. a drip-irrigated coastal orchard, Grismer Evapotranspiration, consumptive use Water inputs to the root zone: et al. (2000) measured rainfall-driven (ETc): Root extraction of available soil Include effective rainfall (the fraction groundwater recharge rates of about 180 water used in plant transpiration. of total rain that infiltrates the soil after millimeters per year in both irrigated and Evapotranspiration, reference (ETo): losses to leaf interception, surface run- fallow areas from 1996 to 1998. Evapotranspiration possible from a tall off, depression storage, and evapora- Clearly, when regional water supply fescue grass crop when there is no limi- tion) and irrigation (in vineyards only allocations are based on water balances tation on available soil water. in this study). that include groundwater resources, rates Groundwater: Soil water stored in Water yield: Runoff from the drain- of groundwater recharge and lag times the root zone, or the combination of age basin, including groundwater to the depths associated with differ- this and deep groundwater below the outflow that appears in the stream ent land uses are critical (Grismer 2012; root zone, stored in water tables or plus groundwater outflow that leaves Sophocleous 2005). Scanlon et al. (2005) aquifers. the basin underground. Roughly, underscored that developing “sustainable Groundwater recharge: In the model, at the basin scale, water yield is the land-uses requires quantitative knowl- infiltration water that exceeds the ca- net precipitation minus the total edge of the linkages between ecosystem pacity of the root zone compartment evapotranspiration. change, recharge and groundwater qual- and percolates to deep groundwater ity.” An effort to understand water use in http://californiaagriculture.ucanr.edu • OctOBER–DEcEMBER 2012 145 oak woodlands or savannas in terms of (2008) and Chen, Rubin et al. (2008) for while ETc for grasslands was about 300 these linkages has been under way during California oak savannas. millimeters. the past several years. Baldocchi et al. (2004) quantified the Miller et al. (2010) recorded ground- rates of canopy evaporation to soil mois- water uptake by California oak trees at Water use in oak woodland, grassland ture in California oak woodland and rates ranging from 4 to 25 millimeters While an understanding of vineyard grasslands and found that ETc rates for per month during June, July and August, water use has developed during the past the grasslands declined when volumet- representing about 80% of total ETc. They few decades, only in the past several ric water content dropped below 15%,