Alexander G. Fernald

Transient storage and hyporheic flow along the Willamette River, Oregon: Field measurements and model estimates

Transient storage is a measure of the exchange of main channel flow with subsurface hyporheic flow and surface water dead zones. Hyporheic flow, in which river water enters the channel bed and banks to reemerge downstream, promotes biochemical processes that are important for water quality and aquatic habitat. Previous studies have quantified transient storage and hyporheic flow on small streams but were not specifically developed to identify both of these processes over long reaches of large rivers. We studied transient storage on the eighth-order upper Willamette River, which flows through high- porosity gravel deposits conducive to hyporheic flow. We used main channel dye tracer studies and solute transport modeling to estimate transient storage on nine study reaches in a 26-km-long study area. We also took dye measurements within the transient storage zone to identify transient storage flow paths. We obtained estimates of transient storage exchange coefficient, αs (mean equals 1.6×10−4 s−1), and transient storage to main channel cross-sectional area, As/A (mean equal to 0.28), that show that significant amounts of water follow flow paths with 0.2–30 hour transient storage zone residence times. Our dye measurements from the transient storage zone itself showed the occurrence of both subsurface and surface flow paths, confirming that hyporheic flow is an important component of estimated transient storage. We found that the two highest As/Aestimates were for reaches that spanned the only length of active main channel in our study area that is unconstrained and where the river can rework large gravel deposits. Much of the natural channel complexity that historically promoted hyporheic flow no longer exists on the upper Willamette River. River management targeting the ecological functions provided by hyporheic flow might best focus on restoring historic hydrogeomorphic processes for creating sites conducive to hyporheic flow.

Hydrologic, Riparian, and Agroecosystem Functions of Traditional Acequia Irrigation Systems

ABSTRACT Traditional cultures in arid landscapes of the southwestern United States and northern Mexico developed irrigation systems to irrigate floodplain valleys along streams and rivers. Many of these traditional irrigation systems, referred to as acequias, continue to be used today. Population growth in the region is creating pressures to convert agricultural land and irrigation water to urban and other uses. Unique hydrologic features of the acequia systems suggest that, beyond providing crop irrigation, they may provide additional valuable hydrologic, riparian, and agroecosystem functions worth maintaining. We investigated in detail the seepage and the groundwater response to seepage from a traditional acequia irrigation ditch along the Rio Grande in north-central New Mexico. We found that 16% of ditch flow seeps into the ditch bed and banks. Groundwater levels near the ditch and midway between the ditch and the river rise 1 m or more within three to four weeks following the start of the irrigation season. The elevated groundwater table produced by ditch and field seepage is sustained until late summer when groundwater levels again drop. The seepage that provides this annual groundwater recharge also sustains riparian vegetation along the main ditch and side ditches. In light of our hydrologic analysis, we considered seepage-supported riparian areas and their ecological functions including aquatic habitat, terrestrial habitat, and water quality effects. Acequia hydrology plays an important role in contributing to an ecologically healthy, agriculturally productive, and community-sustaining floodplain agroecosystem.

Surface Water–Groundwater Interactions Between Irrigation Ditches, Alluvial Aquifers, and Streams

Improved descriptions of surface water–groundwater interactions are required for enhanced water resource management in irrigated areas of the western U.S. We are conducting a research project to determine surface water–groundwater interaction effects on hydrologic budgets and water quality along the upper Rio Grande in New Mexico. This article reports on the initial phase of the project to ascertain the effects of unlined irrigation ditch seepage on shallow groundwater at the Alcalde Science Center in north-central New Mexico. Results from two seepage tests in which 60- and 80 m-long impoundments were established in the Alcalde Ditch indicate that under normal ditch flow depths about 11 cm/day seeps out of the Alcalde Ditch. Based on flow estimates over the 9 km length of the Alcalde Ditch, at least 5% of the total ditch flow seeps out of the ditch bed and banks during the irrigation season. Water level measurements from monitoring wells showed that within 1 month of the beginning of ditch flow, irrigation seepage caused a raised water table and orientation of flow paths towards the river. Specific conductance measurements of surface and shallow groundwater indicated that surface water was the origin of shallow groundwater. Seepage from earthen ditches such as the one in this study could possibly have multiple benefits: diluting agricultural chemicals or septic tank leachate in shallow groundwater, providing groundwater recharge to shallow wells, and providing delayed return flow to the stream thus maintaining in-stream flow after peak runoff periods.

Runoff from simulated rainfall in 2 montane riparian communities.

Riparian ecosystems are the final terrestrial zone before runoff water enters a stream. They provide the last opportunity to decrease non-point source pollution delivery to streams by removing sediments from overland water flow from uplands and roads. To quantify processes of sediment transport, filtration and deposition, it is necessary to determine runoff characteristics for the area. A rotating boom rainfall simulator was used to evaluate the effects of 3 vegetation height treatments (control, 10-cm stubble height, and clipped to the soil surface) in 2 montane riparian plant communities (grass and sedge) on runoff characteristics. Each rainfall simulation event consisted of 2 phases, a dry run of about 60 min followed by a wet run approximately 30 min later. There were no differences in time to runoff initiation for either dry or wet runs that could be attributed to vegetation height treatments for either plant community. It usually required more time for runoff to be initiated in the sedge community compared to the grass community. Generally, there were lower equilibrium runoff percentages from dry runs in the sedge community compared with the grass community. These differences were less during wet runs. Several runoff parameters had characteristics of runoff from water repellent soils. The organic layer on the soil surface exhibited signs of water repellency that reduced the water infiltration rate during the initial stages of a rainfall simulation. These results indicate that runoff and infiltration processes in the surface organic horizon of riparian zones may not respond in the classical manner. This characteristic has important implications if criteria developed in areas with less organic matter on the soil surface are used to manage overland flow in the zone. Additional studies are needed to fully describe infiltration and runoff processes in riparian plant communities.

River Hydrograph Retransmission Functions of Irrigated Valley Surface Water–Groundwater Interactions

Storage and release functions of western U.S. traditional river valley irrigation systems may counteract early and rapid spring river runoff associated with climate variation. Along the Rio Grande in northern New Mexico, we instrumented a 20-km-long irrigated valley to measure water balance components from 2005 to 2007. Hydrologic processes of the system were incorporated into a system dynamics model to test scenarios of changed water use. Of river water diverted into an earthen irrigation canal system, some was consumed by crop evapotranspiration (7.4%), the rest returned to the river as surface return flow (59.3%) and shallow groundwater return flow that originated as seepage from canals (12.1%) and fields (21.2%). The modeled simulations showed that the coupled surface water irrigation system and shallow aquifer act together to store water underground and then release it to the river, effectively retransmitting river flow until later in the year. Water use conversion to nonirrigation purposes and reduced seepage from canals and fields will likely result in higher spring runoff and lower fall and winter river flow.

Deep Percolation and its Effects on Shallow Groundwater Level Rise Following Flood Irrigation

Deep percolation (DP) from irrigation may be important for groundwater recharge in irrigated agricultural river corridors of arid regions, yet few studies of this physiographic setting have characterized both percolation and its direct effects on groundwater levels. The objectives of our study in a sandy loam, flood-irrigated, alfalfa-grass field in northern New Mexico were to (1) compare DP below the 1 m effective root zone based on water balance method (WBM) and Root Zone Water Quality Model (RZWQM) simulations, and (2) characterize effects of DP on shallow groundwater levels. Irrigation water applications were metered, and automated instrumentation measured soil water content and climate data for WBM calculations and RZWQM simulations. Groundwater response was characterized by recorded below-field water levels in four experimental wells. DP varied with initial soil water content and water application amount, ranging from 5 to 18 cm (mean 11.2 ±4.1 SD) with the WBM and from 6 to 17 cm (10.6 ±3.8 SD) with RZWQM (using 0.0005 cm3 cm-3 macroporosity). Across irrigation events, there was high correlation (r = 0.90) between WBM and RZWQM DP. Peak water level response (up to 38 cm) varied from 8 to 16 h after irrigation onset depending on well location and water application amount. Study results show that flood irrigation is a significant source of shallow groundwater recharge. The high correlation between calculated and simulated deep percolation without iterative model calibration indicates that RZWQM can be a useful tool to estimate DP and extend localized field studies to larger spatial scales.

Effect of the Irrigation Canal Network on Surface and Groundwater Interactions in the Lower Valley of the Cachapoal River, Chile

Agricultural production of high value crops in Chiles Central Valley is highly dependent on surface and groundwater resources. They are connected and together form an integrated hydrological system, the individual components of which have to be studied. This research is addressed to answering two questions: 1) to what extent do irrigation and canal seepage contribute to groundwater recharge and 2) what is the influence of the interactions between the Cachapoal River and ground water. The study was carried out from 2003 to 2007 in Peumo Valley (34.3° S, 71.3° W). In winter, the irrigation canal network intercepts and diverts surface runoff, which flows to flat areas and recharges groundwater. In summer, infiltration from the canals recharges the aquifer directly and partially compensates for water uptake from plants and evaporation. The effects of both interactions keep groundwater at a relatively constant level over the whole year. The water balance of the valley is strongly affected by agricultural practices, groundwater recharge mainly originating from irrigation loss (22%) and canal seepage (52%). It is important to know how management decisions, such as change in irrigation practices or canal lining, can affect the hydrological system and agricultural production within the valley.

Shallow Aquifer Recharge from Irrigation in a Semiarid Agricultural Valley in New Mexico

AbstractIrrigation percolation can be an important source of shallow aquifer replenishment in arid regions of the southwestern United States. Aquifer recharge derived from irrigation percolation can be more significant in fluvial valleys overlying shallow aquifers, where highly permeable soils allow rapid water infiltration and aquifer replenishment. This study used data from various irrigation experiments and data at the piezometric level to assess the irrigation percolation effects on the recharge of a shallow aquifer in an agricultural valley of northern New Mexico. The water balance method (WBM) and the water table fluctuation method (WTFM) were used to estimate aquifer recharge at the field scale (<1  ha) and the WTFM was used to determine recharge at the entire valley scale (40  km2). Also, the temporal and spatial distribution of aquifer response to irrigation percolation and canal seepage inputs was characterized. The results showed that for separate irrigation events at the field scale, aquifer r...

Nitrogen dynamics in stream and soil waters.

The mountainous riparian corridor performs important hydrologic functions including nutrient transfers between the terrestrial (upslope) and aquatic (stream) ecosystems. Nitrate-nitrogen and ammonium-nitrogen concentrations were determined on water samples collected in 1993 and 1994 from a montane riparian zone in Northern Colorado. Soil water samples were collected from the riparian corridor and upslope systems, under both losing (summer reservoir releases) and gaining (spring snowmelt runoff) streamflow conditions. Statistical analyses using least square means contrasts were made to identify spatial and temporal differences between: 1) the upslope system and the riparian corridor, 2) the upslope system and the stream, and 3) the riparian corridor and the stream. The Sheep Creek riparian corridor may serve as a sink for nitrate-nitrogen in both gaining and losing streamflow conditions, and as a source for ammonium nitrogen in gaining streamflow conditions. The length of the source or sink period is relatively short and is not meant to suggest differences in site productivity. Streamflow generation mechanisms help determine if the riparian corridor is a nutrient sink or source.

Noncommercial thinning effects on runoff and sediment yield in a mixed conifer New Mexico forest

Programs to thin mixed conifer forests have raised concerns about the potential to increase surface runoff and sediment yield. However, there is little research into the effects of noncommercial thinning of mixed conifer forests on surface runoff and sediment yield. The study objective to evaluate influence of thinning on runoff and sediment yield was accomplished through the use of 60 minute rainfall simulations to measure runoff and sediment yield from 64 1 m2 (10.75 ft2) rings. The experiment was carried out at two landscape positions (ridge and valley) and two aspects (north and south) on paired treated (noncommercial thinning with scattered slash) and untreated plots of 90 × 90 m (295.2 × 295.2 ft) each. Landscape position (valley vs. ridge) significantly affected runoff ratio and sediment yield. Thus, runoff ratio (0.087 se 0.018) and sediment yield (4.54 se 2.69 g m−2 [0.014 se 0.00881 oz ft−2]) were higher at the valley landscape position than the ridge landscape position with a runoff ratio of 0.016 se 0.008 and a sediment yield of 0.418 se 0.27 g m−2 (0.00136 se 0.00885 oz ft−2). This study did not detect significant effects of thinning on time to peak runoff, time to runoff initiation, runoff ratio, and sediment yield.