Print PageThe post-war success in feeding and improving the lives of the growing world population came as a result of humans deliberately engineering agricultural landscapes. Expansion of arable agricultural lands has occurred by engineering the removal of wetlands, via drainage systems, levees, and other structures. Analysis of UN data demonstrates the rate of expansion of arable lands declined after 1990, but agricultural production has kept pace with an estimated 20% increase in population, in part due to wholesale modification and manipulation of the hydrologic cycle. Increased utilization of irrigation is a major factor in maintaining agricultural outputs. There is a correlation between modification of the hydrologic cycle and the loss of ecosystem function, as indicated by the rates of nitrogen flux from major rivers to coastal areas. It is apparent that large-scale modification of the hydrologic cycle has had negative impacts on ecosystem services regionally and globally.
Engineering modification of the hydrologic cycles typically regards only the immediate application (e.g. flood control, water supply), not the need to maintain the function of regional and global ecosystems. The San Joaquin River Valley in California is one of the most highly modified hydrologic regions in the world and restoration efforts are underway to restore wetlands in this landscape. Studies were conducted to develop design criteria for the incorporation of nitrogen removal in mitigation wetlands, which are being constructed for the primary purpose of habitat restoration. Nitrate is the predominate form of nitrogen in agriculturally impacted drainages. Existing mitigation wetlands demonstrated significant, but not large, nitrate-nitrogen mass removal efficiencies. Mean areal nitrate removal rates (J) ranged from 142 to 380 mg-N m-2d-1. First-order rate constants determined for field sites varied between 8.7 and 11.9 cm d-1, but had significant uncertainty (CI95 >57%). Microcosm studies reduced uncertainty and the first order rate constant from the microcosms (9.3 cm d-1) was statistically the same as to the value for the field site. It was determined that between 1.3% and 3.6% of the land in these agricultural watersheds should be managed as wetland to reduce the flux of nitrogen to receiving waters.
Results/Conclusions
Criteria for the engineered design of ecosystem services can and are being developed. Results from combined laboratory and field studies provide engineering criteria for sizing of mitigation wetlands. Results from these studies suggest reasonable modification of the landscape could provide significant removal of nitrogen input to surface waters from highly modifed agricultural landscapes.
