Diuron in surface runoff and tile drainage from two grass-seed fields

The typical method of cool-season grass-seed production in Mediterranean climates briefly exposes surface waters to potentially high concentrations of the herbicide diuron [3-(3,4-dichlorophenyl)-1,1-dimethyl urea] during the initial season of growth. To better understand the process, and the degree, of diuron transport from agricultural fields, two grass-seed fields in the Willamette Valley of Oregon were monitored for diuron loss in surface runoff and tile drainage during the first wet season after planting. Initial diuron concentrations in surface runoff were high (>1000 microg L(-1) in one field and >100 microg L(-1) in the other), though they decreased by two orders of magnitude by the end of the season. Concentrations in the tile drains were as much as 1000 times lower than in the surface runoff during the first few weeks of runoff events, and they remained lower than surface water concentrations throughout the season. Total losses in surface runoff were between 1.3 and 3% of the amount applied-much higher than losses via the tile drains. It is also shown by means of a simple first-order decay model that, when little information is available, it may be best to describe diuron depletion in runoff water as a function of cumulative rainfall during the wet season.     

Estimation of urban sensible heat flux using a dense wireless network of observations

The determination of the sensible heat flux over urban terrain is challenging due to irregular surface geometry and surface types. To address this, in 2006–07, a major field campaign (LUCE) took place at the école Polytechnique Fédérale de Lausanne campus, a moderately occupied urban site. A distributed network of 92 wireless weather stations was combined with routine atmospheric profiling, offering high temporal and spatial resolution meteorological measurements. The objective of this study is to estimate the sensible heat flux over the built environment under convective conditions. Calculations were based on Monin–Obukhov similarity for temperature in the surface layer. The results illustrate a good agreement between the sensible heat flux inferred from the thermal roughness length approach and independent calibrated measurements from a scintillometer located inside the urban canopy. It also shows that using only one well-selected station can provide a good estimate of the sensible heat flux over the campus for convective conditions. Overall, this study illustrates how an extensive network of meteorological measurements can be a useful tool to estimate the sensible heat flux in complex urban environments.     

Migration of saline solutions in variably saturated porous media

Migration of concentrated NaNO3 solutions in homogeneous packs of pre-wetted silica sands was investigated using a light transmission system. Solutions of 5 molal NaNO3 were found to migrate downward 24-62% faster than pure water, in an unstable, fingered manner. This behavior was attributed primarily to a surface tension induced, non-zero apparent contact angle between the imbibing and the resident fluids. For saline solutions of similar surface tension to that of pure water (achieved by the addition of 2% methanol), the migration rates and plume shapes were comparable to that of water, demonstrating that density was not the primary source of the observed differences in migration patterns. At depths where resident saturation increased above residual, the migration process appeared to occur via film flow with slight changes in saturation (<4%), rather than in a series of abrupt jumps, as observed at shallower depths. A method for contact angle scaling was used to illustrate the effects of non-zero contact angles on capillary pressure-saturation curves.     

EVALUATION OF PROBABILITY DENSITY FUNCTIONS IN PRECIPITATION MODELS FOR THE PACIFIC NORTHWEST1

ABSTRACT: Recent work has found that a one-parameter Weibull model of wet day precipitation amount based on the Weibull distribution provides a better fit to historical daily precipitation data for eastern U.S. sites than other one-parameter models. The general two-parameter Weibull distribution was compared in this study to other widely used distributions for describing the distribution of daily precipitation event sizes at 99 sites from the U.S. Pacific Northwest. Surprisingly little performance was sacrificed by reducing the two-parameter Weibull to a single-parameter distribution. Advantages of the single-parameter model included requiring only the mean wet day precipitation amount for calibration, invertibility for simulation purposes, and ease of analytical manipulation. The fit of the single-parameter Weibull to the 99 stations included in this study was significantly better than other single-parameter models tested, and performed as well as the widely endorsed, more cumbersome, two-parameter gamma model. Both the one-and two-parameter Weibull distributions are shown to have b-moments that are consistent with historical precipitation data, while the ratio of b-skew and b-variance in the gamma model is inconsistent with the historical recerd by this measure. In addition, it was found that the two-parameter gamma distribution was better fit using the method of moments estimators than maximum likelihood estimates. These findings suggested that the distribution in precipitation among sites in the Pacific Northwest with dramatically different settings are nearly identical if expressed in proportion to the mean site event size.     

Discussion: “Meadow Restoration Increases Baseflow and Groundwater Storage in the Sierra Nevada Mountains of California” by Luke J.H. Hunt,Julie Fair,and Maxwell Odland

We discuss a recent paper which evaluated the hydrologic changes resulting from a pond‐and‐plug meadow restoration project in the Sierra Nevada Mountains of California. In the study, measurements of streamflow into and out of the meadow suggested late‐summer baseflow increased as much as five‐fold when compared with prerestoration conditions. However, the volume of streamflow attributed to the restored meadow (49,000–96,000 m³ over four months) would require that 2.5–4.8 m of saturated meadow soils drain during summer months. The groundwater data from this meadow record only 0.45 m of change over this timeframe, which is less than might be expected from plant use alone (0.75 m), suggesting this restored meadow may be acting as a water sink throughout summer rather than a source.     

The influence of land-cover changes on the variability of saturated hydraulic conductivity in tropical peatlands

Mitigation and Adaptation Strategies for Global Change - Understanding the movement of water through peat is essential for effective conservation and management strategies for peatlands. Saturated...