Ecosystem services (ES) provided by ditches in a desert agricultural valley

Agricultural ditches can be an efficient method for removing nutrients in wetlands and rivers. In this paper, we document that additionally, ditches offer other ecosystem services (ES) such as providing bird habitat and sediment retention. A typical ditch reach of 6 km with variable width between 3 and 80 m and water depth from 0.06 and 0.5 m, was selected as a monitoring site among 1492 km of a drain network in the agricultural Mexicali Valley located in the Colorado River Delta. Nitrogen and phosphorus concentration and discharge flow rate determinations were obtained monthly during a year, at three stations along the ditch, dividing the reach into two segments. Total nutrient removal efficiency of 39%, along the monitoring year was observed. Additionally, nutrient mass flow had a significant winter-spring seasonal increment associated with crop irrigation, up four times for total nitrogen, nearly 2.5 times for nitrogen-based nutrients, and five times for PO4. Monthly estimations of normalized difference vegetation index (NDVI) from Sentinel images were obtained to evaluate the response of riparian vegetation to high flow discharge seasonal fluctuations, from 0.072 to 1.219 m3 s-1, and nutrient mass flow changes. No correlation among them was found, as riparian species are tolerant to high water levels and nutrient concentration fluctuation. Downstream changes in water electrical conductivity from 6.06 to 8.45 mS cm-1 along the resulted in the shift from Typha domingensis to Phragmatis australis which has a higher salinity tolerance. An intensive bird census showed ditch vegetation provided suitable habitat and refuge to 29 different bird species, including five common birds of the flooded remnant areas of the Colorado River Delta, as well as migratory birds. By using slope criteria of drainage network, 47% of it has a high and moderated potential of sediment retention. These ES are fundamental in semiarid regions because of the intense land-use changes from riparian and desert ecosystems to irrigation lands, as well as water being diverted from natural streams to irrigation channels. This study highlights the importance of considering agricultural ditches as helpful, not only as a natural water treatment of agricultural pollutants, but also as providers of resident and migratory bird habitat and vegetation diversity and erosion regulation by sediment retention on desert agricultural valleys.