All Issues

In California's semi-arid climate, replenishment of groundwater aquifers relies on precipitation and runoff during the winter season. However, climate projections suggest more frequent droughts and fewer years with above-normal precipitation, which may increase demand on groundwater resources and the need to recharge groundwater basins. Using historical daily streamflow data, we developed a spatial index and rating system of high-magnitude streamflow availability for groundwater recharge, STARR, in the Central Valley. We found that watersheds with excellent and good availability of excess surface water are primarily in the Sacramento River Basin and northern San Joaquin Valley. STARR is available as a web tool and can guide water managers on where and when excess surface water is available and, with other web tools, help sustainable groundwater agencies develop plans to balance water demand and aquifer recharge. However, infrastructure is needed to transport the water, and also changes to the current legal restrictions on use of such water.

Alternaria late blight (ALB), caused mainly by the fungal pathogen Alternaria alternata, is an important pistachio disease that causes severe tree defoliation and fruit shell staining. Its control relies on multiple fungicide sprays, including carboxamide fungicides. In 2015, we surveyed 35 orchards representing nine pistachio producing counties of California to determine the current situation of Alternaria resistance to four widely used carboxamide fungicide active ingredients. This survey showed that isolates collected in the northern (Tehama, Glenn and Colusa counties) and southern (Tulare, Kings and Kern counties) Central Valley presented higher frequencies of carboxamide resistance than isolates collected from orchards in the central region (Fresno, Madera and Merced counties). The number of carboxamide usages in a year is the main factor determining elevated resistance. By extracting the A. alternata DNA and sequencing the carboxamide target genes, we evaluated the prevalence of specific molecular alterations (mutations) associated with carboxamide fungicide resistance. Finally, we identified cross-resistance patterns among different carboxamide fungicides, leading to recommendations about combinations to avoid.

We used the Revised Universal Soil Loss Equation (RUSLE) to evaluate how different residual forage dry matter (RDM) levels affect erosion potential in rangelands across California. The model was adapted to operate in a geographic information system (GIS) to model 14.8 million acres (6.0 million hectares) of land. Average erosion potential was low among all RDM scenarios and increased from an estimated 0.05 ton per acre per year (0.11 megagram per hectare per year) with the high RDM scenario to 0.12 ton per acre per year (0.27 megagram per hectare per year) with the low RDM scenario. Considering all RDM scenarios, fewer than 174,733 acres (70,710 hectares, or 1.2% of land) had erosion potential that exceeded soil loss tolerance values. Although achieving a uniform RDM target across a landscape may be an oversimplification of reality, simulations suggest that erosion potential on average is low in California's annual rangelands across high, moderate and low RDM recommendations. Moreover, our findings indicate that grazing management (maintaining moderate or high RDM) to mitigate erosion can be effective when targeted at areas of high vulnerability.