Sources and Transport of Phosphorus to Rivers in California and Adjacent States, U.S., As Determined by Sparrow Modeling1
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JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION Vol. 51, No. 6 AMERICAN WATER RESOURCES ASSOCIATION December 2015 SOURCES AND TRANSPORT OF PHOSPHORUS TO RIVERS IN CALIFORNIA AND ADJACENT STATES, U.S., AS DETERMINED BY SPARROW MODELING1 Joseph Domagalski and Dina Saleh2 ABSTRACT: The SPARROW (SPAtially Referenced Regression on Watershed attributes) model was used to sim- ulate annual phosphorus loads and concentrations in unmonitored stream reaches in California, U.S., and por- tions of Nevada and Oregon. The model was calibrated using de-trended streamflow and phosphorus concentration data at 80 locations. The model explained 91% of the variability in loads and 51% of the variabil- ity in yields for a base year of 2002. Point sources, geological background, and cultivated land were significant sources. Variables used to explain delivery of phosphorus from land to water were precipitation and soil clay content. Aquatic loss of phosphorus was significant in streams of all sizes, with the greatest decay predicted in small- and intermediate-sized streams. Geological sources, including volcanic rocks and shales, were the princi- pal control on concentrations and loads in many regions. Some localized formations such as the Monterey shale of southern California are important sources of phosphorus and may contribute to elevated stream concentra- tions. Many of the larger point source facilities were located in downstream areas, near the ocean, and do not affect inland streams except for a few locations. Large areas of cultivated land result in phosphorus load increases, but do not necessarily increase the loads above those of geological background in some cases because of local hydrology, which limits the potential of phosphorus transport from land to streams. (KEY TERMS: nutrients; phosphorus; geology; transport and fate; simulation; watersheds; SPARROW.) Domagalski, Joseph and Dina Saleh, 2015. Sources and Transport of Phosphorus to Rivers in California and Adjacent States, U.S., as Determined by SPARROW Modeling. Journal of the American Water Resources Associ- ation (JAWRA) 51(6):1463-1486. DOI: 10.1111/1752-1688.12326 INTRODUCTION recognized as especially important because of the glo- bal demand for phosphorus fertilizers (Cordell et al., 2009; Carpenter and Bennett, 2011), and problems, Phosphorus is a naturally occurring element that such as eutrophication, caused by runoff or erosion of occurs in the earth’s crust within rocks and soils, and soils from agricultural fields (Vollenweider, 1968). in all living organisms. The average crustal abun- The global biogeochemical cycle of phosphorus dance of phosphorus is 0.1% (Rudnick, 2003; Canfield includes inputs to soils through chemical weathering et al., 2005). Phosphorus is one of 16 elements neces- of phosphorus-containing minerals, cycling through sary for plant growth, and is a primary nutrient terrestrial and aquatic ecosystems, runoff to the along with nitrogen and potassium (Mullins, 2009). oceans and associated biogeochemical processes, and The management of phosphorus use in water- sedimentation. Tectonic processes such as volcanism sheds, and particularly agricultural watersheds, is and uplift move phosphorus to continental systems 1Paper No. JAWRA-13-0243-P of the Journal of the American Water Resources Association (JAWRA). Received November 8, 2013; accepted September 25, 2014. © 2015 American Water Resources Association. This article is a U.S. Government work and is in the public domain in the USA. Discussions are open until six months from issue publication. 2Research Hydrologist (Domagalski and Saleh), California Water Science Center, U.S. Geological Survey, 6000 J Street, Placer Hall, Sacramento, California 95819 (E-Mail/Domagalski: [email protected]). JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 1463 JAWRA DOMAGALSKI AND SALEH under geologic time frames (Compton et al., 2000). loads. SPARROW models are useful for a manage- Unlike nitrogen, there is essentially no atmospheric ment context because of the knowledge gained on component of the global phosphorus cycle, except by how constituents are sourced and transported in the dust. The terrestrial and aquatic biogeochemical unmonitored catchments. Several SPARROW models cycles are complicated by interactions between living of phosphorus from other large watersheds have been material and charged mineral surfaces in soils or sed- previously published (Brown et al., 2011; Garcia iments, such as iron or manganese oxides, which sorb et al., 2011; Moore et al., 2011; Wise and Johnson, phosphorus and limit its bioavailability (Compton 2011). et al., 2000) and transport to aquifers. The problems associated with phosphorus use have prompted studies or tools to assess specific risks. These tools use soil phosphorus levels, application STUDY AREA AND METHODS methods, tillage, soil erodibility, and other factors to determine and mitigate the risks (Lemunyon and Gilbert, 1993; Eghball and Gilley, 2001). Although Location and Geology those types of tools have value, they do not necessar- ily address how much phosphorus may be transported The study area included most of the state of Cali- on the watershed or catchment level. Management fornia and portions of adjacent Oregon and Nevada considerations designed to lower phosphorus concen- (Figure 1). The study area is made up of 17 six-digit trations or yields in specific river systems must take hydrological unit code regions (HUC6). A listing of into account the broader context of processes, includ- these HUCs, and their areas, is shown in Table 1. ing geological sources, affecting transport in order to The boundaries of the six-digit HUCs are shown in develop realistic water quality goals. Phosphorus is Figure 1. There are several large rivers of manage- one of many different constituents on the 303(d) list of ment or ecological importance in the study area. The impaired water bodies mandated under the Clean largest river, the Sacramento River (Figure 1), is the Water Act for protection of the beneficial uses of major source of freshwater to the Delta (Figures 1 water. In 2010, there were 192 listings for phosphorus and 2) of the Sacramento and San Joaquin Rivers. of impaired water bodies on the California 303(d) list Water from the Sacramento River is used for agricul- (http://www.waterboards.ca.gov/water_issues/programs/ ture and is a source of drinking water (CADWR, tmdl/integrated2010.shtml). The listings include 42 2009a, b). A large portion of the entire study area different streams or reservoirs located throughout the drains to the Delta, making source identification for state. nutrients an important management tool. The Delta, In this study, the SPARROW (SPAtially Refer- along with the rest of the San Francisco Bay system, enced Regression on Watershed attributes) model is the largest estuary on the North American west was used to provide a regional understanding of coast, and also very important to California water phosphorus sources and transport in most of Califor- use and the state’s economy (http://www.water.ca. nia and portions of adjacent Oregon and Nevada. gov/swp/delta.cfm). The San Joaquin River has con- SPARROW is a hybrid statistical and mechanistic siderably less discharge relative to the Sacramento model for estimating the movement of the mass of a River, but is also a major source of nutrients to the particular constituent through the landscape under Delta. Other large rivers include the Klamath, Rus- long-term steady-state conditions (Schwarz et al., sian, Eel, Salinas, Merced, Stanislaus, Tuolumne, 2006; Preston et al., 2009). This model utilizes and numerous others that drain to the Pacific Ocean. watershed characteristics including sources of the Selected rivers are shown in Figure 1 and their rele- modeled constituent, land use or land cover, climate, vance to the model and to phosphorus loads trans- and stream properties to explain the spatial variation ported to coastal regions is discussed later in this in measured mean annual stream load. The SPAR- manuscript. Discharge on most of the major rivers of ROW model provides interpretations relevant to an the study area result from a combination of natural “average” year in a region, and does not capture the and managed flow. There are a number of reservoirs seasonality of stream load, nor can it capture the present throughout the study area that supply water loadings that would occur during storm events. for agriculture, urban, and environmental uses, and Streamflow and water quality data are de-trended, as provide flood protection. Most of the larger reservoirs explained in the methods section to capture long-term are situated in undeveloped or relatively undeveloped average conditions. A modification to the SPARROW areas, and there are also many smaller ones located model is being tested that would incorporate tran- in mixed land use areas. sient conditions, that is using nonaveraged or de- There are three “Level-Three” ecological regions as trended data, and be able to distinguish seasonal defined by the U.S. Environmental Protection Agency JAWRA 1464 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION SOURCES AND TRANSPORT OF PHOSPHORUS TO RIVERS IN CALIFORNIA AND ADJACENT STATES, U.S., AS DETERMINED BY SPARROW MODELING 124°0'0"W R. a th 120°0'0"W 42°0'0"N K lam Sco 180102 t t Sal . m R lo R . C u d o c n M 180202 R Pit R. T . rinit y R. 180800 S a Eel c R. r a Oregon m Cr. 180201 e e t n t California Nevada 180101 t u o B R . kee R. uc R T r 160501 u Legend ss ia n Lake Tahoe R Calibration Site . 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