The lack of major effects in either season reflect the nature of the modeling scenario: recall that these islands are treated as “pre-flooded”— with salinity levels set the same as the surrounding channels; this corresponds to long-term conditions or near-term flooding under high river-flow conditions within the Delta, not the near-term effect of a “Big Gulp” of saltwater that might occur if the islands flood in the summer or fall or a very dry winter or spring. The contrast between the irrigation and non-irrigation seasons may reflect the effects of the D-1641 regulations , which include requirements to maintain low “X2” salinity standards in the western Delta from February until June. As a result, water exporters responded by increasing pumping in the fall for storage and urban uses, drawing more saline water toward the pumps. Permanent flooding of western islands greatly increases the volume of flood tide inflows and reduces the ability of the out-flowing water to restrain salinity intrusion.Sea level rise leads to limited crop revenue losses in the Delta, both with dual conveyance and through Delta exports . Dual conveyance for Delta exports generally increases total revenue losses somewhat relative to through-Delta exports, but these losses remain well under 1% of total revenues. During dry years, when Delta waters are more saline, dual export conveyance gives the highest revenue losses, slightly above 0.7% ,maceta 30 litros roughly $4.5 million yr-1, with most losses occurring in the western Delta.
Results using the RMA 2-D hydrodynamic modeling for salinity with permanently flooded western islands also show little revenue loss during the irrigation season. Some areas in the north of the Delta may even see slight decreases in water salinity and corresponding increases in crop revenues. Because most salinity changes occur outside the main irrigation season, crop yield and revenue effects are largely confined to acreage planted to winter crops such as wheat; thus the absolute revenue losses are very small because the acreage of winter plantings is itself small . Of course, beyond their effects on water salinity in the Delta, the permanent flooding of the five western islands would also lead to losses from flooded land being taken out of production. Elsewhere, we used DAP to show that farm revenue losses from the permanent flooding of 19 western and central Delta islands would far exceed the salinity-related losses shown here—roughly $66 to $90 million yr-1—10% or more of baseline crop revenues . Changes of this magnitude would also ripple through the regional economy , causing additional losses in revenues and value added. Large conversions of farmland to habitat could also have more substantial local and regional economic effects than the salinity changes modeled here .Because the historical 1981–2000 water year, average export of 5.96 billion m3 yr-1 might not be representative of more recent, higher export levels, we replicated Fleenor et al. WAM hydrodynamic modeling runs using the 1996–2005 water years, when average exports were 7.28 billion m3 yr-1 . We found no major increase in salinity for any of the 52 sampling stations considered during the irrigation season. The largest increase was 2% at the Mokelumne River station near Terminous Tract, and the average electrical conductivity across all stations in the Delta was generally lower than during the 1981–2000 period. This is because the 1996–2005 period was fully covered in the D-1641 requirements in operation from the mid-1990s; under these requirements, the isohaline line of 2ppt must be maintained in the far western Delta from February to June to support delta smelt.
During the non-irrigation season of the 1981–2000 time period, however, dual conveyance may increase salinity in some areas, including those near Old and Middle Rivers, which are intake points to supply Delta water for urban uses in Contra Costa Water District. The change in salinity in these locations is about 15%, which would increase water treatment costs for the Contra Costa Water District service area if the utility were unable to store water during lower salinity periods for later use. To test the sensitivity of the DAP agro-economic model, we also examine cases with uniform values of irrigation water salinity for all islands at 1%, 3%, 5% and 10% of seawater salinity or 33 ppt. DAP responds more abruptly to electrical conductivity levels beyond one percent of seawater. This analysis supports conclusions from earlier modeling : crop revenue losses from salinity increases caused by dual conveyance and sea level rise are relatively low because most higher value crops are not located in parts of the Delta that experience the highest salinity increases. When identical, higher levels of irrigation water salinity are assumed for all Delta islands and sub regions, DAP reports generally higher agricultural revenue losses than those shown in the previous section. This results from two factors: first, the proportional salinity increases are much higher on islands and sub regions in the Delta where salinity is currently low; second, these areas also tend to have greater concentrations of higher-value crops. Thus, increased salinity conditions and losses of higher value crops increase revenue losses substantially . The absolute revenue losses are highest in the northern and southern Delta, where such higher value crops predominate .Human activities associated with food and energy production have greatly elevated nitrogen bio-availability to an extent that exceeds the assimilative capacity in many terrestrial ecosystems, often leading to large increases in N fluxes to waters . Enrichment of N leads to eutrophication of surface waters causing degradation of aquatic ecosystems, such as toxic algal blooms, low dissolved oxygen, depletion of fish populations, and loss of aquatic biodiversity .
To effectively guide watershed management to control N pollution, it is essential to quantify the response of riverine N export to changes in sources and levels of anthropogenic N inputs . Nitrogen budgets are useful for evaluating impacts of human activities on the N cycle by relating anthropogenic N inputs to outputs . Net anthropogenic nitrogen input is a budgeting approach that sums annual N contributions from atmospheric deposition, fertilizer application, agricultural fixation, seed input, and netimport/export in feed and food . The NANI approach has been applied to many watersheds acrossAsia , America , and Europe . It is a simple yet powerful approach to evaluate net N inputsfrom anthropogenic sources to terrestrial ecosystems, as well as an effective toolto explain among-watershed or among-year variations in riverine N exports. However, the relationship between NANI and riverine N export is additionally influenced by variations in hydroclimate and land management activities, as well asprogressive Nsaturation of terrestrial ecosystems . It is commonly observed that years with higher precipitation or river discharge export a higher fraction ofNANI via rivers than drier years . Furthermore, the export fraction of NANI via rivers can be enhanced by improved agricultural drainage systems . Previous studies also demonstrate a larger fractional export of NANI by rivers when NANI exceeds some threshold value , which corresponds to NANI exceeding the N assimilative capacity of terrestrial and aquatic ecosystems . As a result, changes of climate, land management,macetas cuadradas plastico and the degree of N saturation have a strong potential to enhance riverine N export . Importantly, the influence of climate change, land-use change and progressive N saturation is difficult to detect from short-term records, instead requiring a long-term record of NANI and riverine N export dynamics. For a watershed, the NANI components of chemical fertilizer, atmospheric deposition, biological fixation and seed input are the primary N inputs to forest and agricultural systems, while residential systems mainly receive N from human and animal wastes. In terms of N delivery pathways, N exports from forest and agricultural landscapes to the river network are mainly via diffuse runoff and leaching , while a portion of N from residential systems enters the river network via direct sewage discharge . In addition, the greater impervious surface area in residential systems further enhances N delivery efficiency . As a result, residential systems have a higher potential to export NANI than forest/agricultural systems. This is especially true in developing countries where agricultural subsurface drainage and efficient treatment of residential wastewater are both often lacking . Therefore, it is valuable to separate watershed forest/agricultural and residential N budgets to effectively identify their contrasting export fractions and relative contributions to riverine N fluxes. Based on extensive data collection for three adjacent catchments subjected to low, medium and high levels of anthropogenic impacts in eastern China, this study provides a long-term analysis of the response of riverine TN export to changes in forest/agricultural and residentialN budgets,landuse and climate.
Specifically, this study examines temporal and spatial variations of NANI to forest/agricultural and residential systems, addresses temporal and spatial variations of riverine N fluxes; develops empirical models for linking NANIFA and NANIR to riverine TN fluxes, and identifies individual contributions from NANIFA, NANIR and other sources to the riverine TN flux. This study improves the NANI budgeting methodology to separately estimate watershed NANIFA and NANIR budgets andidentifies of their contributions to annual riverine TN flux. Such quantitative knowledge is essential for managers to determine which systems and sources should be targeted for N reduction.The three catchments in this study are located in the rapidly developing Taizhou region of Zhejiang Province, China . The three rivers are tributaries of the Jiaojiang River, which is the third largest river of Zhejiang Province and flows into Taizhou Estuary and the East China Sea, a coastal area that commonly experiences hypoxia . The climate is subtropical monsoon having an average annual temperature of 17.2 C andaverage annual precipitation of 1395 mm. Due to the dominant volcanic, malmstone and mudstone bedrock lithologies, groundwater or base flow contributes 45% of annual river water discharge in the upper Jiaojiang River watershed the investigation made by local Hydrology Bureau. Rainfall mainly occurs in May– September with a typhoon season in July–September, while winter is a major dry season . From 1980 to 2010, there were no significant trends in annual precipitation or river discharge in the three catchments . Considering the availability of relevant long-term data, this study selected three catchments within the Jiaojiang watershed to provide a range of anthropogenic impacts from agricultural activity and urbanization . In terms of managed land area percentage , as well as agricultural drainage, human population and domestic animal density, the three catchments were classified into low , medium and high levels of anthropogenic impacts. Except for catchment HD, catchments LD and MD have comparable slopes and altitudes due to their locations within the watershed .Between 1980 and 2010, average managed land area percentage was 4%, 9% and 26% for catchments LD, MD and HD, respectively. Correspondingly, natural forest and barren lands accounted for 96%, 91% and 74% of the entire catchment area for LD, MD and HD, respectively. Thus, LD may be considered as a reference catchment for examining progressive nitrogen saturation in a dominantly natural ecosystem over the study period. The average percentage of agricultural lands with efficient drainage systems was 12%, 14% and 35% for LD, MD, and HD, respectively . Average population density was 74, 134 and 761 capita km2 , while domestic animal density was 36, 65, and 153 capita km2 for LD, MD, and HD, respectively. Over the 31-year study period, human population within LD, MD, and HD increased by 25%, 32%, and 30%, while managed land area increased by 39%, 43%, and 24%, respectively. The economic role of agriculture has been increasingly replaced by industry since the 1990s, resulting in a remarkable reduction in cropland cultivation as well as N fertilizer use since 2000 . Due to reduction in cropland and increasing availability of chemical fertilizer, recycled animal and human excreta for fertilizing croplands also decreased from 93% in 1980 to 21% in 2010 . The remaining animal wastes were removed via artificial treatments and direct discharge to the environment . Agricultural land area irrigated and drained with improved cement channels and pipes increased by 96–108% since 1980 .Reducing tradeoffs among ecosystem services related to nitrogen cycling in agriculture is a global challenge. One promising strategy to reduce such tradeoffs relies on a stronger role for biological processes to support high yields, such as practiced in organic agriculture, rather than non-renewable inputs like synthetic N fertilizer.