Paired Watershed Research Articles

Effects of slope aspect, grazing, and sampling position on the soil penetration resistance curve

The slope aspect, grazing intensity, and vegetation canopy are factors that affect the physical and mechanical properties of soil. However, their effects on the soil penetration resistance (PR) curve are unclear. Thus, we investigated the effects of northern and southern slope aspects, the grazing intensity (free and controlled grazing, and livestock exclusion), and the sampling position (beneath and between canopy) on the PR curve and bulk density in the Gonbad paired watershed in Hamedan, Iran. At the end of the grazing season, we collected and analyzed seven undisturbed samples (total=252) and one disturbed soil sample (total=36) from each experimental unit. The soil PR was measured at different suctions. We fitted the To and Kay (2005) and Stock and Downes (2008) models to the PR data. The effects of treatments on the bulk density, PR curve and its model coefficients, and PR were analyzed at different suctions. The slope aspect had the greatest impact on the soil bulk density and PR at suctions of 30 and 1500kPa. The Stock and Downes (2008) model coefficients, which represent the degree of change in the PR relative to the moisture content, were affected mostly by the sampling position. Southern aspects had higher overall PR curves than northern aspects due to their lower soil organic matter contents and more coarse fragments. The overall PR curve increased with the grazing intensity. There were interactions between the effects of the three factors on the PR curve and all affected the PR.

Yellow Perch (Perca flavescens) Mercury Unaffected by Wildland Fires in Northern Minnesota.

Wildland fire can alter mercury (Hg) cycling on land and in adjacent aquatic environments. In addition to enhancing local atmospheric Hg redeposition, fire can influence terrestrial movement of Hg and other elements into lakes via runoff from burned upland soil. However, the impact of fire on water quality and the accumulation of Hg in fish remain equivocal. We investigated the effects of fire-specifically, a low-severity prescribed fire and moderate-severity wildfire-on young-of-the-year yellow perch () and lake chemistry in a small remote watershed in the Boundary Waters Canoe Area Wilderness in northeastern Minnesota. We used a paired watershed approach: the fire-affected watershed was compared with an adjacent, unimpacted (reference) watershed. Prior to fire, upland organic horizons in the two study watersheds contained 1549 μg Hg m on average. Despite a 19% decrease in upland organic horizon Hg stocks due to the moderate severity wildfire fire, fish Hg accumulation and lake productivity were not affected by fire in subsequent years. Instead, climate and lake water levels were the strongest predictors of lake chemistry and fish responses in our study lakes over 9 yr. Our results suggest that low- to moderate-severity wildland fire does not alter lake productivity or Hg accumulation in young-of-the-year yellow perch in these small, shallow lakes in the northern deciduous and boreal forest region.

Forest cover change and water yield in large forested watersheds: A global synthetic assessment

AbstractThe effects of forest cover change on water yield have long been studied across the globe. Several reviews have summarized the impacts of forest change and water yield from the small and paired watershed experiments, but no any synthetic assessment has been conducted on the basis of studies of large watersheds (>1,000 km2). We conducted a synthetic analysis on the basis of the studies from 162 large studied watersheds across the globe to explore how forest cover change affects annual water yield. Our first‐ever assessment confirms that deforestation increases annual water yield and reforestation decreases it, which is consistent with results from paired watershed experiments. More importantly, we found that forest cover and climate variability play a coequal role in annual water yield variations. The effects of forest cover change and climate variability on annual water yield variations can be additive or offsetting. Thus, their interactions can critically determine the magnitudes and directions of water yield changes. We also found that the hydrological sensitivities to forest cover change in smaller and dryer watersheds are higher than those in larger and wetter ones. The implications of these findings for sustainable water and watershed management are discussed in the context of future land cover and climate changes.

Water balance in paired watersheds with eucalyptus and degraded grassland in Pampa biome

Rangelands of the Pampa biome, which cover regions of Argentina, Uruguay and Brazil (176,496km2 – 2.07% of Brazilian territory and 63% of Rio Grande do Sul State territory, southern region of Brazil) in South America (total area of 750,000km2), are being substituted by crops and commercial eucalyptus, with potential impacts on ecological and hydrological response of watersheds and river basins. We evaluated the influence of vegetation cover on hydrological processes by describing the water balance and its components (rainfall – P, interception, throughfall, actual evapotranspiration – ETa, groundwater recharge – G, and streamflow – Q) in two paired watersheds. One watershed being cropped to 7-year-old Eucalyptus saligna stands (forest watershed – FW; 0.83km2) and another consisted of degraded grassland with native and exotic grasslands (grassland watershed – GW; 1.10km2) used for livestock production in the Rio Grande do Sul State. The study was conducted from October 2012 to September 2014, during two hydrological years: a normal year from October 2012 to September 2013, and a wetter year from October 2013 to September 2014. Pluviometers were installed to study the partition of rain, along with watershed gauges and equipment for monitoring hydrological variables. Meteorological data (maximum and minimum temperature, relative humidity, wind speed, and solar radiation) used to calculate potential evapotranspiration were collected from a tower installed in the FW, whereas hydrological data (P and Q) were collected by sensors installed in each of the watershed spillways. During the normal year, P was 19% above the annual historical average for the region, which is 1314mm, whereas in wetter year, rainfall was 98% above the same average. Total rainfall interception was similar between years in GW (9 and 10%), but different between years in FW that was higher in wetter (24%) then in normal year (16%). In the normal year, streamflow were 64% lower in the FW compared to the GW, while ETa and G were respectively 37% and 25% greater in the FW compared to the GW. In the wetter year, streamflow was 66% lower in the FW than in the GW, while ETa and G in soil were respectively 27% and 46% greater in the FW compared to the GW. Flow with 5% time streamflow (Q5) was greater in the GW compared to FW in both normal and wetter years. Streamflow in the GW and FW were equal at Q80 and Q82 in the normal and in the wetter years, respectively, and exceedance probability curves crossed over at Q81 and Q82, where the exceedance probability curves become greater in FW than in the GW. Even if the forest watershed had greater ETa compared to the grassland watershed, benefits such as greater interception and lesser surface runoff can be highlighted, for a condition where grassland was degraded and provided low aboveground biomass. Thus, the cultivation of eucalyptus stands may provide better structural conditions and ground cover, greater infiltration and soil water retention, and increased groundwater recharge, with consequent reduction of soil degradation by erosion and increased water availability during dry periods. Long-term use of forest systems, especially when compared to degraded grassland, may provide improvement on soil physical quality. However, these comparative results may not be valid for conditions during harvesting and tillage operations, neither when soil under grasslands has improved physico-hydraulic properties.

Water Quality Effects of Switchgrass Intercropping on Pine Forest in Coastal North Carolina

Abstract. Interplanting a cellulosic bioenergy crop (switchgrass, L.) between loblolly pine ( L.) rows could potentially provide a sustainable source of bio-feedstock without competing for land currently in food production. The objectives of this study were to: (1) quantify the concentrations and loads of drainage water nitrogen (N) and phosphorus (phosphate) associated with establishment and growth of switchgrass treatments and compare them with those for a mid-rotation pine forest (control), and (2) quantify the treatment effects on drainage water N and phosphate (IC) and switchgrass only (SG). Thinned mid-rotation loblolly pine with natural understory (MP) was used as the control. Pretreatment calibration equations for nutrients were obtained using a paired watershed approach and bootstrap geometric regression with 2007-2008 data, when pine on all sites had reached canopy closure. Treatment effects were calculated as the difference between expected values from the pretreatment relationship and measured data for the treatment period. Precipitation, outflow, and N and phosphate concentrations in the outflow were measured during calibration (Jan. 2007 to Dec. 2008), site preparation for switchgrass establishment (Nov. 2009 to Mar. 2012), and switchgrass growth (Apr. 2012 to Apr. 2014). Mean NO3-N concentrations and loads were significantly (a = 0.05) greater for SG than for IC during the switchgrass growth period. Average treatment concentrations with standard errors and total load effects during switchgrass growth for NO3-N followed the trends SG (-0.002 ±0.01 mg L-1) > IC (-0.12 ±0.04 mg L-1) and SG (0.75kg ha-1) > IC (0.23kg ha-1), respectively. For phosphate average concentrations and loads, the treatment effects during switchgrass growth followed the trends SG (-0.004 ± mg L-1) >IC ( -0.02 ± mg L-1) and IC (-0.43 kg ha-1) > SG (-0.70 kg ha-1), respectively. Average concentration effects for NO3-N and phosphate and total load effects for phosphate significantly (a = 0.05) decreased for IC compared to the MP control. These results suggest that the intercropping treatment (IC) with loblolly pine and switchgrass improved water quality by reducing NO3-N and phosphate concentrations and phosphate loads. Keywords: Bioenergy crop, Bootstrap geometric regression, Loblolly pine, Nutrients, Paired watershed.

Effects of Controlled Drainage on Water Table Recession Rate

Abstract. Controlled drainage is a best management practice that decreases nitrate loads from subsurface drainage, but questions remain about optimal operation strategies. One unanswered question is whether the outlet should be lowered prior to or directly after a rainfall event to reduce the amount of time that the water table is at a level that would be detrimental to either trafficability or crop yield. The objective of this study was to determine how much controlled drainage lengthens the time needed for the water table to fall after a rainfall event, to inform possible improvement in the management of controlled drainage systems. This objective was addressed using water table recession rates from two pairs of controlled and free-draining fields located at the Davis Purdue Agricultural Center in Indiana over a period of nine years (2006-2014). At each pair, comparison of mean recession rates from the two fields indicated that controlled drainage reduced recession rate. The significance of the relationship between paired observations and the effect of controlled drainage was determined by a paired watershed approach using analysis of variance (ANOVA) and covariance (ANCOVA). Raising the outlet of the subsurface drainage system decreased the mean rate of water table recession by 29% to 62%, increasing the time needed for the water table level to fall from the surface to 30 and 60 cm depths by approximately 12 to 26 h and 24 to 53 h, respectively. Based on these results, it can be concluded that lowering the outlet before storm events would reduce the amount of time that the water table is at a detrimental level for either crop growth or trafficability. However, the trade-off between costs and benefits of active management depends on the sensitivity of the crop and probability of a severe storm. Keywords: Drainage water management, Managed drainage, Paired watershed approach, Tile drainage, Water table drawdown.

Assessment of wildland fire impacts on watershed annual water yield: Analytical framework and case studies in the United States

AbstractMore than 50% of water supplies in the conterminous United States originate on forestland or rangeland and are potentially under increasing stress as a result of larger and more severe wildfires. Little is known, however, about the long‐term impacts of fire on annual water yield and the role of climate variability within this context. We here propose a framework for evaluating wildland fire impacts on streamflow that combines double‐mass analysis with new methods (change point analysis, climate elasticity modeling, and process‐based modeling) to distinguish between multiyear fire and climate impacts. The framework captures a wide range of fire types, watersheds characteristics, and climate conditions using streamflow data, as opposed to other approaches requiring paired watersheds. The process is illustrated with three case studies. A watershed in Arizona experienced a +266% increase in annual water yield in the 5 years after a wildfire, where +219% was attributed to wildfire and +24% to precipitation trends. In contrast, a California watershed had a lower (−64%) post‐fire net water yield, comprised of enhanced flow (+38%) attributed to wildfire offset (−102%) by lower precipitation in the post‐fire period. Changes in streamflow within a watershed in South Carolina had no apparent link to periods of prescribed burning but matched a very wet winter and reports of storm damage. The presented framework is unique in its ability to detect and quantify fire or other disturbances, even if the date or nature of the disturbance event is uncertain, and regardless of precipitation trends.

Long-Term (13 Years) Decomposition Rates of Forest Floor Organic Matter on Paired Coniferous and Deciduous Watersheds with Contrasting Temperature Regimes

Two sets of paired watersheds on north and South facing slopes were utilized to simulate the effects of temperature differences that are on the scale of those expected with near-term climatic warming on decomposition. Two watersheds were pine plantations (Pinus strobus L.) and two were mature deciduous forests established at similar elevation ranges and precipitation at the Coweeta Hydrologic Laboratory, but they differed in slope aspect (north vs. South facing), solar radiation, and litter temperature by about 2.0 °C. Nylon netting was placed on plots each year for 13 years and litterfall was measured. This time span in which decomposition rate was measured encompassed the time until less than 8% of the initial C remained. Decomposition rates of foliar litter were significantly faster on the slightly warmer watersheds, in both the coniferous and deciduous forests (Analysis of Variance). The turnover rate (year−1) was 0.359 (±0.006) for the South facing vs. 0.295 (±0.011) for the North facing coniferous watersheds, and 0.328 (±0.011) vs. 0.297 (±0.012) for the corresponding deciduous watersheds. Turnover rates of pine vs. deciduous broadleaf litter over 13 years were not significantly different because of the high proportion of relatively refractory Quercus spp. in the deciduous litterfall and because of a trend towards convergence of the rates after two years. After a greater decomposition rate in the first year or two, years 2–13 fit a negative exponential curve well (a timespan not well represented in literature) and there was only a small accumulation of humus older than 13 years. The fate of C in litterfall in the South facing deciduous forest was as follows: 14.3% was lost as leaching of dissolved organic C, 2.2% was lost as downward fine particulate matter flux from the bottom of the forest floor, 78.2% was mineralized (by mass balance), leaving only 5.4% of foliar litter after 13 years of decomposition. In these soils with a mor type O horizon, there was evidence that translocation of DOC and in-situ root production must be more important sources of mineral soil organic matter than downward migration of particulate humus.

Qualitative characteristics of water resulting from the introduction of Eucalyptus silviculture in Pampa biome, RS

ABSTRACT This study aimed to present the effect on surface water quality of the introduction of eucalyptus forestry in areas that were traditionally used for extensive cattle farming in the Pampa biome, by comparing two paired watersheds located in the municipality of Rosário do Sul, one of them used for forestry and the other in an anthropized natural grassland condition in the Pampa biome. For this purpose, every fifteen days the following parameters were collected and analyzed in two watersheds with different land uses (watershed with grassland and extensive livestock farming – GW and watershed with Eucalyptus - EW) between the months of August 2011 and August 2012: pH, EC, temperature, turbidity and concentrations of SO42-, K+, Ca2+, Mg2+, Cl-, BOD5,20, SS, DS, alkalinity, total coliforms and Escherichia coli. Water quality and land use parameters averages were compared using the t-Test to account for the land use and seasonality. It was concluded that the introduction of forestry activity together with the areas of environmental protection required by the Brazilian legislation (Permanent Preservation Areas - PPA plus Legal Reserve - LR), contributed to the increased concentrations of conductivity, dissolved solids, alkalinity and calcium, and the decreased concentrations of total coliforms and Escherichia coli.

Improving model prediction reliability through enhanced representation of wetland soil processes and constrained model auto calibration – A paired watershed study

Process based, distributed watershed models possess a large number of parameters that are not directly measured in field and need to be calibrated, in most cases through matching modeled in-stream fluxes with monitored data. Recently, concern has been raised regarding the reliability of this common calibration practice, because models that are deemed to be adequately calibrated based on commonly used metrics (e.g., Nash Sutcliffe efficiency) may not realistically represent intra-watershed responses or fluxes. Such shortcomings stem from the use of an evaluation criteria that only concerns the global in-stream responses of the model without investigating intra-watershed responses. In this study, we introduce a modification to the Soil and Water Assessment Tool (SWAT) model, and a new calibration technique that collectively reduce the chance of misrepresenting intra-watershed responses. The SWAT model was modified to better represent NO3 cycling in soils with various degrees of water holding capacity. The new calibration tool has the capacity to calibrate paired watersheds simultaneously within a single framework. It was found that when both proposed methodologies were applied jointly to two paired watersheds on the Delmarva Peninsula, the performance of the models as judged based on conventional metrics suffered, however, the intra-watershed responses (e.g., mass of NO3 lost to denitrification) in the two models automatically converged to realistic sums. This approach also demonstrates the capacity to spatially distinguish areas of high denitrification potential, an ability that has implications for improved management of prior converted wetlands under crop production and for identifying prominent areas for wetland restoration.

Quantification of Hydrologic Response of Staggered Contour Trenching for Horti‐pastoral Land Use System in Small Ravine Watersheds: A Paired Watershed Approach

AbstractA paired watershed approach was used to quantifying the effects of staggered contour trenching (SCT) densities on runoff and soil loss from a long‐term field experiment. Four adjacent physiographical similar ravine watersheds were selected. After 2‐year calibration period, three of the four watersheds were treated with different SCT densities and leaving one watershed as untreated control (WC). The SCT densities of 139, 278 and 417 trenches ha−1 were determined based on the targeted 25, 50 and 75% runoff trapping potential of the WT1, WT2 and WT3 treatment watersheds, respectively. Runoff and soil loss were recorded for a period of 11 years including 2 years of calibration period. The calibration and treatment prediction equations for the treatment watershed with respect to untreated control watershed were developed by the simple linear regression. F‐test and t‐test (p < 0·0001) suggested that calibration predictions were highly significant and treatment prediction equations differ significantly. Results showed mean runoff reductions of 86·1, 60·5 and 37·7% in the WT3, WT2 and WT1 treatment watershed over the control, respectively. Similar trends as the runoff were also recorded in soil loss reduction by 124·9, 77·1 and 40·0% for the WT3, WT2 and WT1 over Wc, respectively. Paired t‐test (p < 0·001) indicated that the means of the treatment effects on runoff and soil loss in treatment watersheds differ significantly with control watershed. It was concluded that 417 staggered contour trenches ha−1 is an optimum trenching density for supporting the horti‐pastoral land use system in ravine lands of India. Copyright © 2016 John Wiley & Sons, Ltd.

Streamflow response to increasing precipitation extremes altered by forest management

AbstractIncreases in extreme precipitation events of floods and droughts are expected to occur worldwide. The increase in extreme events will result in changes in streamflow that are expected to affect water availability for human consumption and aquatic ecosystem function. We present an analysis that may greatly improve current streamflow models by quantifying the impact of the interaction between forest management and precipitation. We use daily long‐term data from paired watersheds that have undergone forest harvest or species conversion. We find that interactive effects of climate change, represented by changes in observed precipitation trends, and forest management regime, significantly alter expected streamflow most often during extreme events, ranging from a decrease of 59% to an increase of 40% in streamflow, depending upon management. Our results suggest that vegetation might be managed to compensate for hydrologic responses due to climate change to help mitigate effects of extreme changes in precipitation.

A regional estimate of postfire streamflow change in California

AbstractThe effect of fire on annual streamflow has been examined in numerous watershed studies, with some studies observing postfire increases in streamflow while other have observed no conclusive change. Despite this inherent variability in streamflow response, the management of water resources for flood protection, water supply, water quality, and the environment necessitates an understanding of postfire effects on streamflow at regional scales. In this study, the regional effect of wildfire on annual streamflow was investigated using 12 paired watersheds in central and southern California. A mixed model was used to pool and statistically examine the combined paired‐watershed data, with emphasis on the effects of percentage area burned, postfire recovery of vegetation, and postfire wetness conditions on postfire streamflow change. At a regional scale, postfire annual streamflow increased 134% (82%–200%) during the first postfire year assuming 100% area burned and average annual wetness conditions. Postfire response decreased with lower percentages of percentage area burned and during subsequent years as vegetation recovered following fire. Annual streamflow response to fire was found to be sensitive to annual wetness conditions, with postfire response being smallest during dry years, greatest during wet years, and slowly decreasing during very wet years. These findings provide watershed managers with a first‐order estimate for predicting postfire streamflow response in both gauged and ungauged watersheds.

Evaluating hydrologic effects of spatial and temporal patterns of forest canopy change using numerical modelling

Although hydrologic responses to land cover changes are often studied using a paired watershed approach, it is not feasible to assess the hydrological effects of many different patterns of land cover alteration by empirical studies alone. An alternative is to use well validated, spatially explicit, physically based numerical models to estimate watershed storage and flux dynamics. The objectives of this study were to assess the sensitivity of watershed flow regimes to several spatial and temporal patterns of forest harvest and recovery in a snow-dominated mountain watershed. The Distributed Hydrology Soil-Vegetation Model (DHSVM) was parameterized using 1998–2007 climate data for the 28-km2 Mica Creek Experimental Watershed (MCEW), a headwater catchment in the inland Pacific Northwest. The modelling experiment indicated that clear-cutting the entire watershed would increase runoff volume by 79% and 5th percentile flows by 68%. Hydrologic recovery resulting from forest regeneration after clear-cut harvesting is expected to take up to 25 years to return to baseline conditions, and 50 years to fully recover to preharvest conditions. A more realistic harvesting scenario where the watershed was gradually harvested in a series of clear-cut blocks allowing for subsequent regeneration to occur was also assessed. This approach reduced the magnitude of hydrologic alteration. Analysis of several other scenarios, defined by aspect, elevation, and distance to the stream network, revealed that flow regime was more sensitive to the amount of alteration rather than pattern and landscape position of disturbance. Copyright © 2015 John Wiley & Sons, Ltd.

Comparing the influence of wildfire and prescribed burns on watershed nitrogen biogeochemistry using 15N natural abundance in terrestrial and aquatic ecosystem components.

We evaluated differences in the effects of three low-severity spring prescribed burns and four wildfires on nitrogen (N) biogeochemistry in Rocky Mountain headwater watersheds. We compared paired (burned/unburned) watersheds of four wildfires and three spring prescribed burns for three growing seasons post-fire. To better understand fire effects on the entire watershed ecosystem, we measured N concentrations and δ15N in both the terrestrial and aquatic ecosystems components, i.e., soil, understory plants in upland and riparian areas, streamwater, and in-stream moss. In addition, we measured nitrate reductase activity in foliage of Spiraea betulifolia, a dominant understory species. We found increases of δ15N and N concentrations in both terrestrial and aquatic ecosystem N pools after wildfire, but responses were limited to terrestrial N pools after prescribed burns indicating that N transfer from terrestrial to aquatic ecosystem components did not occur in low-severity prescribed burns. Foliar δ15N differed between wildfire and prescribed burn sites; the δ15N of foliage of upland plants was enriched by 2.9 ‰ (difference between burned and unburned watersheds) in the first two years after wildfire, but only 1.3 ‰ after prescribed burns. In-stream moss δ15N in wildfire-burned watersheds was enriched by 1.3 ‰, but there was no response by moss in prescription-burned watersheds, mirroring patterns of streamwater nitrate concentrations. S. betulifolia showed significantly higher nitrate reductase activity two years after wildfires relative to corresponding unburned watersheds, but no such difference was found after prescribed burns. These responses are consistent with less altered N biogeochemistry after prescribed burns relative to wildfire. We concluded that δ15N values in terrestrial and aquatic plants and streamwater nitrate concentrations after fire can be useful indicators of the magnitude and duration of fire effects and the fate of post-fire available N.

Calibration of SWAT model for woody plant encroachment using paired experimental watershed data

Summary Globally, rangeland has been undergoing a transition from herbaceous dominated grasslands into tree or shrub dominated woodlands with great uncertainty of associated changes in water budget. Previous modeling studies simulated the impact of woody plant encroachment on hydrological processes using models calibrated and constrained primarily by historic streamflow from intermediate sized watersheds. In this study, we calibrated the Soil and Water Assessment Tool (SWAT model), a widely used model for cropping and grazing systems, for a prolifically encroaching juniper species, eastern redcedar (Juniperus virginiana), in the south-central Great Plains using species-specific biophysical and hydrological parameters and in situ meteorological forcing from three pairs of experimental watersheds (grassland versus eastern redcedar woodland) for a period of 3-years covering a dry-to-wet cycle. The multiple paired watersheds eliminated the potentially confounding edaphic and topographic influences from changes in hydrological processes related to woody encroachment. The SWAT model was optimized with the Shuffled complexes with Principal component analysis (SP-UCI) algorithm developed from the Shuffled Complexes Evolution (SCE_UA). The mean Nash–Sutcliff coefficient (NSCE) values of the calibrated model for daily and monthly runoff from experimental watersheds reached 0.96 and 0.97 for grassland, respectively, and 0.90 and 0.84 for eastern redcedar woodland, respectively. We then validated the calibrated model with a nearby, larger watershed undergoing rapid eastern redcedar encroachment. The NSCE value for monthly streamflow over a period of 22 years was 0.79. We provide detailed biophysical and hydrological parameters for tallgrass prairie under moderate grazing and eastern redcedar, which can be used to calibrate any model for further validation and application by the hydrologic modeling community.

Comparing nitrate sink strength in perennial filter strips at toeslopes of cropland watersheds.

Integration of perennial filter strips (PFS) into the toeslopes of agricultural watersheds may decrease downstream nitrate (NO) losses. However, long-term NO removal depends on the relative importance of several NO sinks in the PFS. Plant biomass and labile soil organic matter (SOM) are temporary NO sinks, while stable SOM is a long-term, but potentially finite, NO sink. In contrast, denitrification is a permanent NO sink. We investigated the relative importance of these NO sinks in PFS at the toeslope of row crop watersheds in Iowa. Using 25- × 30-cm in situ mesocosms, we added NO to PFS soils and quantified NO-N recovery in plant biomass and SOM after one growing season. Further, we compared NO-N recovery in particulate (relatively labile) and mineral-associated (relatively stable) SOM in mesocosms with and without growing perennial vegetation. To determine the potential importance of denitrification, we compared denitrification enzyme activity in soils from paired watersheds with and without PFS. Transfer of NO-N into labile and stable SOM pools was rapid and initially independent of growing vegetation. However, SOM and plant biomass were both relatively minor NO sinks, accounting for <30% of NO-N inputs. Denitrification enzyme activity data indicated that dissolved organic carbon derived from perennial vegetation increased potential denitrifier activity in PFS soils compared with row crop soils. Together, these results constrain SOM and plant biomass as NO sinks and indicate that denitrification was the most important NO sink in perennial filter strips over one growing season.

Short-term hydrological responses to silvicultural treatments withina stream buffer zone: a case study

The thinning of stream buffer zones (SBZs) is gaining popularity as a silvicultural management practice in order to reduce the risk of wildfire and insect hazard, provide economic return, and improve the effectiveness of SBZs. In this study, streamflow over a 1-year period was monitored at 2 small paired watersheds (treated vs. reference). The short-term impacts of a partial cutting within a SBZ as well as the relative effects of pastoral, forested, and clearcut areas on changes in downstream hydrology were examined. Upstream pastoral areas had a higher water yield than downstream forested sections during the no-harvest (calibration) period of 6 months. The partial cut (about 50% of the basal area) within the SBZ changed the hydrologic pattern by remarkably increasing the water yield on the treated downstream sections during the 6-month-long postharvest period. The harvest operation also caused an increase in direct runoff at 2 downstream sections, WT2 and WT3 (~200% and ~100%, respectively). No significant changes were observed in the water yield pattern of the control watershed. Because harvested areas within the SBZs constitute a fraction of the monitored sub-watersheds and only partial harvesting (~50% of the basal area) was implemented, the observed increase in flow at the treated downstream sections (100% and 250%, respectively) is unprecedented. The partial harvesting within the SBZs also resulted in a significantly flashier hydrological system. Because silvicultural treatments are part of regular, repeated management operations, these short term (in this study, 6-month-long) but substantial changes in water yield, direct runoff, and flashiness could have important implications for water quality, water resources, and downstream biota.

Using observations and a distributed hydrologic model to explore runoff thresholds linked with mesquite encroachment in the Sonoran Desert

AbstractWoody plant encroachment is a world wide phenomenon with implications on the hydrologic cycle at the catchment scale that are not well understood. In this study, we use observations from two small semiarid watersheds in southern Arizona that have been encroached by the velvet mesquite tree and apply a distributed hydrologic model to explore runoff threshold processes experienced during the North American monsoon. The paired watersheds have similar soil and meteorological conditions, but vary considerably in terms of vegetation cover (mesquite, grass, bare soil) and their proportions with one basin having undergone mesquite removal in 1974. Long‐term observations from the watersheds exhibit changes in runoff production over time, such that the watershed with more woody plants currently has less runoff for small rainfall events, more runoff for larger events, and a larger runoff ratio during the study periods (summers 2011 and 2012). To explain this observation, we first test the distributed model, parameterized with high‐resolution (1 m) terrain and vegetation distributions, against continuous data from an environmental sensor network, including an eddy covariance tower, soil moisture, and temperature profiles in different vegetation types, and runoff observations. We find good agreement between the model and observations for simultaneous water and energy states and fluxes over a range of measurement scales. We then identify that the areal fraction of grass (bare soil) cover determines the runoff response for small (large) rainfall events due to the dominant controls of antecedent wetness (hydraulic conductivity). These model‐derived mechanisms explain how woody plants have differential effects on runoff in semiarid basins depending on precipitation event sizes.

Conversion of Conservation Tillage to Rotational Tillage to Reduce Phosphorus Losses during Snowmelt Runoff in the Canadian Prairies.

In a preceding study, converting conventional tillage (ConvT) to conservation tillage (ConsT) was reported to decrease nitrogen (N) but to increase phosphorus (P) losses during snowmelt runoff. A field-scale study was conducted from 2004 to 2012 to determine if conversion of ConsT to rotational tillage (RotaT), where conservation tillage was interrupted by a fall tillage pass every other year, could effectively reduce P losses compared with ConsT. The RotaT study was conducted on long-term paired watersheds established in 1993. The ConvT field in the pair has remained under ConvT practice since 1993, whereas tillage was minimized on the ConsT field from 1997 until 2007. In fall 2007, RotaT was introduced to the ConsT field, and heavy-duty cultivator passes were conducted in the late fall of years 2007, 2009, and 2011. Runoff volume and nutrient content were monitored at the edge of the two fields, and soil and crop residue samples were taken in each field. Greater soil Olsen P and more P released from crop residue are likely the reasons for the increased P losses in the ConsT treatment (2004-2007) relative to the ConvT treatment (2004-2007). Analysis of covariance indicated that, compared with ConsT (2004-2007), RotaT (2008-2012) increased the concentrations of dissolved organic carbon (DOC) by 62%, total dissolved N (TDN) by 190%, and total N (TN) by 272% and increased the loads of DOC by 34%, TDN by 34%, and TN by 60%. However, RotaT (2008-2012) decreased soil test P in surface soil, P released from crop residue, and duration of runoff compared with ConsT (2004-2007) and thus decreased the concentrations of total dissolved P (TDP) by 46% and total P (TP) by 38% and decreased the loads of TDP by 56% and TP by 42%. In the Canadian Prairies, where P is a major environmental concern compared with N, RotaT was demonstrated to be an effective practice to reduce P losses compared with ConsT.