Fig. 1.

The geographic location and land use of the coastal Salinas Valley and the inland Tulare Lake Basin of California (left) and spatially distributed groundwater nitrate (NO3) loading [kg N ha−1 yr−1] from all systems considered (right) for the 1945 (top) and 2005 (bottom) periods. About half of all manure N in the study area is generated and applied in Tulare County, which causes significant applications of manure N in excess of typically applied N.


Fig. 2.

Flows of nitrogen (N) for calculation of groundwater loading. The groundwater loading from cropland (E) was calculated by mass balance (E = B + J + G + L − A − FL − FH − D). The groundwater loading from alfalfa (Ealf) and manure management on livestock facilities (I) was estimated from rates reported in the literature for California. The groundwater loading from human systems (K) was calculated from a mixture of literature values and data reported to regulatory agencies.


Fig. 3.

Historical agricultural development in the study area (not including alfalfa): total harvested cropland area (overlying blue line), fertilizer applied to that cropland (yellow line), manure applied to that cropland (brown line), sum of manure and fertilizer applied to cropland (dark red line), and total harvested N in fertilized crops (shown in green bars) based on cropland areas reported by California County Agricultural Commissioner Crop Reports (County crop reports for the years studied—1943–1947, 1958–1962, 1973–1977, 1988–1992, 2003–2007—can be found at the California Department Food and Agriculture Website,;Table 1).