View Full Table | Close Full ViewTable 1.

Historical changes in the total harvested area (ha) in the study area counties, by crop group (Agricultural Commissioner Crop Reports†). Values shown are the median of 5 yr of annual data for each period (year indicates the central year of the period). Areas harvested more than once (typical for some vegetable and field crops in 1990 and 2005) due to planting two crops in 1 yr are counted more than once. Thus, actual area on the ground (California Augmented Multisource Landcover, Fig. 1) is smaller. Crop groups, corn, and forage are totaled across the study area counties before taking medians for each 5-yr period. County totals are for median crop areas for each county in each 5-yr period. Hence, the study area totals for crop groups and for county are not identical. Forage includes alfalfa, grain and hay, and corn, which is part of field crops.

 
Crop group or county 1945 1960 1975 1990 2005
ha
Alfalfa 118,260 210,165 165,247 155,788 169,373
Cotton 104,796 275,464 393,782 429,732 246,810
Field crops 54,179 99,175 129,095 105,304 131,538
Grain and hay 210,651 353,793 304,459 161,263 223,468
Rice 3,148 10,197 7,790 2,686 2,098
Grapes 107,967 99,743 131,150 152,613 155,385
Nuts 3,412 5,879 32,463 71,170 136,717
Subtropical 21,100 20,691 58,475 68,055 88,420
Tree fruit 25,155 25,535 27,978 45,657 61,719
Vegetables and berries 75,809 90,490 132,626 209,524 280,433
Total‡ 606,217 980,967 1,217,818 1,246,004 1,326,588
 Corn 1,841 19,389 28,279 42,739 106,619
 Forage 320,062 586,738 479,721 340,917 485,040
Fresno County 240,580 399,498 411,882 414,473 419,776
Kern County 133,432 203,677 281,909 306,623 304,182
Kings County 91,217 139,898 211,786 174,020 162,744
Monterey County 98,771 97,678 102,833 107,409 150,998
Tulare County 117,878 174,475 219,161 245,659 276,492
Total 681,877 1,015,348 1,227,570 1,248,183 1,314,191
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, http://www.cdfa.ca.gov/exec/county/countymap/
Excludes alfalfa.



View Full Table | Close Full ViewTable 2.

Five-yr period median total annual nitrogen (N) typically applied [Gg N yr−1] in the study area for five historic periods and reductions needed in the future to meet groundwater loading rates <35 kg N ha−1 yr−1, by crop group, based on typical rates for each crop and based on the area shown in Table 1. Values do not include excess manure or effluent and other incidental N (atmospheric or irrigation water) applied to cropland, shown in the last column.

 
Crop group 1945† 1960 1975 1990 2005 2005 avg. N input reductions needed‡ 2005 avg. county exported (excess) manure N rate§
Gg N yr−1 % kg ha−1 yr−1
Alfalfa 1.42 3.57 3.64 1.87 2.03 0–10
Cotton 6.60 25.3 47.3 82.1 47.1 39 20–100
Field crops 3.34 8.90 16.5 18.4 29.0 0 20–110
Grain and hay 11.0 26.0 30.8 25.2 42.1 8 20–100
Rice 0.16 0.74 0.74 0.38 0.30 18 20–70
Grapes 1.19 1.70 2.89 5.65 5.75 0 0–20
Nuts 0.72 1.08 6.35 13.15 25.32 31 20–90
Subtropical fruit 3.84 3.09 10.3 7.10 9.21 26 10–50
Tree fruit 3.63 3.03 3.72 5.19 7.09 58 10–60
Vegetables and berries ¶ 6.50 11.7 22.1 44.7 59.2 38 20–110
All crops except alfalfa 37.0 81.6 140.6 201.2 225.1
The year indicated represents the central year of a 5-yr period; for example, 1945 refers to the 1943–1947 period, and 2005 refers to the 2003–2007 period.
Relative reduction of synthetic fertilizer inputs needed, by crop group, to reduce 2005 loading to 35 kg N ha−1 yr−1. The model conservatively assumes that yields are maintained at current levels. Higher reductions would be needed otherwise. These reductions do not include reductions to also remove excess manure N applied (last column).
§Excess N application from manure during the 2005 period is simulated by distributing the county total manure exported from dairies across all county crop acreage outside of dairies proportional to each crop’s typically applied N. The range indicates the minimum and maximum rates computed across the five counties in the study area.
Total N applied in all crops are slightly different than shown in Table 3 because the basis for cropped area here are the Agricultural Commissioner Crop Reports (ACRs) reported areas (Table 1) not the spatial California Augmented Multisource Landcover data (Fig. 1), which survey land use by different methods than ACRs.



View Full Table | Close Full ViewTable 3.

Summary of simulated, spatially distributed N fluxes based on California Augmented Multisource Landcover land areas that account for groundwater loading from nonalfalfa cropland (1.39 million ha in 2005), alfalfa (130,000 ha in 2005), dairy facilities, wastewater, and urban lawns and golf courses from 1945 to 2005. The N mass balance components for calculation of the nonalfalfa cropland groundwater loading are also shown. All fluxes shown in Gg N yr−1. Letters correspond to flows depicted in Fig. 2.

 
Nonalfalfa cropland mass balance component 1945 1960 1975 1990 2005
Gg N yr−1
Nitrogen inputs
 H. Synthetic fertilizer N applied on cropland after accounting for manure use as fertilizer 43.7 93.8 155.2 210.7 230.1
 J. Land applied manure
  On dairy (only after mid-1960s) 32.0 42.2 48.0
  Off dairy (only after 1980) 6.9 38.7
  Long-term farm soil N storage (recovery loss after 1980) 6.9 38.7
 G. Biosolids and wastewater effluent N 2.0 3.1 2.5 2.9 5.7
 B. Atmospheric N deposition (cropland without alfalfa) 5.0 10.2 15.6 15.7 13.5
 L. Irrigation water N (groundwater nitrate) 0 4.3 10.6 15.4 21.4
Nitrogen outputs [Gg N yr−1]
 F. Harvested N from cropland 25.0 65.0 94.2 110.4 139.6
 D. Surface runoff N from cropland 9.9 15.4 19.4 18.1 19.3
 A. Atmospheric N losses from cropland 5.1 11.1 21.6 29.4 35.8
Groundwater loading
 E. From nonalfalfa cropland 10.7 19.9 80.7 136.0 162.7
 Ealf. From alfalfa cropland 3.7 6.6 4.5 3.8 4.5
 I. From dairy facilities (corrals and lagoons) 1.7 1.7 1.7
 K. From human systems
  Wastewater† 1.0 1.5 2.4 3.7 4.6
  Urban lawns and golf courses 0.2 0.4 1.0 1.6 1.8
  Septic systems 0.6 1.0 1.2 1.8 2.3
Other relevant fluxes
 Synthetic N applied on nonalfalfa cropland, unadjusted‡ 44.2 94.8 160.7 221.7 242.2
 Synthetic N applied on alfalfa 1.5 3.7 3.3 1.5 1.8
 Dairy manure N flux to cropland before 1970s 7.7 15.8
 Incidental to grazed pasture at 135 kg N ha−1 yr−1 4.2 9.9
 Applied to grain crops 3.5 5.9
Wastewater includes leaky sewer systems and percolation from wastewater treatment plant and food processing percolation basins.
As outlined in Section “Synthetic Fertilizer Use.” The rate shown is before adjusting for manure or effluent and biosolids that are assumed to be used in lieu of synthetic N to meet typical application rates in dairy- or facility-specific known land application areas.