Pasture Catchment Research Articles

Efficacy of an off-line constructed wetland for mitigating nitrogen loss from a pastoral catchment

ABSTRACT Constructed wetlands are an established mitigation option for reducing contaminant loads from pastoral land in New Zealand. We assess the long-term performance of a 0.5 ha, off-line, horizontal surface flow wetland, installed on a dairy farm with the primary objective of nitrate removal. Over seven years the median removal rates for nitrate-nitrogen (NO3–N), total nitrogen and total suspended solids were 45%, 33% and 74%, respectively. Whilst dissolved reactive phosphorus (DRP) was initially removed, over time the wetland progressed to a small exporter of DRP. Median NO3–N mass removal was 417 kg yr−1 , which is considerable for such a small wetland, and equivalent to 7% of total modelled ‘farm-scale’ losses. Nitrate removal was found to be seasonal and although it could be simulated using a conventional wetland model with Arrhenius temperature dependence, the resulting joint estimates of k 20 and Θ were uncertain. We estimate the NO3–N removal cost-effectiveness of the wetland (over a 25-year lifespan) to be NZ$34 kg N−1. We foresee opportunities for off-line constructed treatment wetlands to be an effective nitrate mitigation option in perennial drains and lowland streams in pastoral catchments, but also emphasise the need for on-going maintenance to ensure longevity of performance.

The ability of detainment bunds to decrease sediments transported from pastoral catchments in surface runoff

AbstractErosion leading to sedimentation in surface water may disrupt aquatic habitats and deliver sediment‐bound nutrients that contribute to eutrophication. Land use changes causing loss of native vegetation have accelerated already naturally high erosion rates in New Zealand and increased sedimentation in streams and lakes. Sediment‐bound phosphorus (P) makes up 71–79% of the 17–19 t P y−1 delivered from anthropogenic sources to Lake Rotorua in New Zealand. Detainment bunds (DBs) were first implemented in the Lake Rotorua catchment in 2010 as a strategy to address P losses from pastoral agriculture. The bunds are 1.5–2 m high earthen stormwater retention structures constructed across the flow path of targeted low‐order ephemeral streams with the purpose of temporarily ponding runoff on productive pastures. The current DB design protocol recommends a minimum pond volume of 120 m3 ha−1 of contributing catchment with a maximum pond storage capacity of 10 000 m3. No previous study has investigated the ability of DBs to decrease annual suspended sediment (SS) loads leaving pastoral catchments. Annual SS yields delivered to two DBs with 20 ha and 55 ha catchments were 109 and 28 kg SS ha−1, respectively, during this 12‐month study. The DBs retained 1280 kg (59%) and 789 kg (51%) of annual SS loads delivered from the catchments as a result of the bunds' ability to impede stormflow and facilitate soil infiltration and sediment deposition. The results of this study highlight the ability of DBs to decrease SS loads transported from pastures in surface runoff, even during large storm events, and suggests DBs are able to reduce P loading in Lake Rotorua.

The influence of land cover and storm magnitude on hydrologic flowpath activation and runoff generation in steep tropical catchments of central Panama

Despite abundant research documenting that land use/land cover (LULC) have substantial impacts on the hydrology of humid tropical systems, field-based evidence for the physical mechanisms behind these impacts are still lacking. In particular, our understanding of the hydrologic flowpaths that generate runoff in these systems, and how they vary with respect to LULC is insufficient to inform both physically-based hydrologic modeling and land-use decision-making. In this study, we use end-member mixing analysis (EMMA) of stream chemistry, and hydrometric characterizations of hillslope soil moisture to identify hydrologic flowpaths in humid tropical steep-land catchments of varying LULC: mature tropical forest, young secondary tropical forest, cattle pasture. EMMA was applied to data from 14 storm events (six at the mature forest, five at the young secondary forest, and three at the cattle pasture) that were intensively sampled during the 2017 wet season representing a wide range of rainfall magnitudes and intensities. Additionally, volumetric-soil-moisture responses at multiple depths were characterized during and after 74 storm events occurring from 2015 to 2017. EMMA results indicated that lateral preferential flow within the top 30 cm of the soil profile was a dominant source of runoff generation at the two forested catchments, with the contribution of this flow path increasing with rainfall magnitude and intensity. This was corroborated by volumetric-soil-moisture data, that showed that a perched zone of saturation developed at 30 cm at the time of peak storm runoff during the largest events and lasted for the remaining duration of the event. EMMA indicated that runoff was a combination of infiltration-excess overland flow and lateral subsurface flow in the actively grazed pastoral catchment. There, overland flow contributed 62 % of runoff during the highest runoff rate sampled (35.3 mm/hr) and this contribution increased substantially with storm magnitude. This flowpath identification was also supported by volumetric-soil-moisture data at the pasture, with peak saturation at all depths during the largest storm events occurring up to 30 min after peak runoff. These results provide a mechanistic explanation for previously observed hydrologic differences among tropical LULCs. Additionally, the wide range of hydrologic conditions during these storm events provide a basis for understanding how future changes to this, and similar humid tropical regions will impact hydrological processes and water availability.

Precipitation Characteristics and Land Cover Control Wet Season Runoff Source and Rainfall Partitioning in Three Humid Tropical Catchments in Central Panama

AbstractMechanisms of runoff generation in the humid tropics are poorly understood, particularly in the context of land‐use/land cover change. This study analyzed the results of 124 storm hydrographs from three humid tropical catchments of markedly different vegetation cover and land‐use history in central Panama during the 2017 wet season: actively grazed pasture, young secondary succession, and near‐mature forest. We used electrical conductivity to separate baseflow (old water) from storm‐event water (new‐water). In all three land covers, new‐water dominated storm runoff generation in 44% of the sampled storm events, indicating the dominance of fast shallow flow paths in the landscape. Activation of these flow paths was found to depend on a combination of maximum rainfall intensity and total storm rainfall, which, in turn, relates to markedly contrasting hydrograph separation results among land covers. Relationships between these rainfall characteristics and storm runoff generation were nonlinear, producing a threshold response with the exceedance of specific rainfall volumes and/or intensities. The pastoral catchment delivered order of magnitude more new‐water during storm events than the two forested catchments. Although new‐water contributed minimally (<10%) to total wet season runoff in the forested catchments, 43% of runoff generation in the pasture came from five large rainfall events where a threshold response produced substantial increases in total runoff and new‐runoff efficiency. Based on our results, we propose a conceptual model of hydrologic flow paths in humid tropical systems that can explain previously observed disparities in seasonal storage and runoff with respect to land use/land cover.

Native woodland establishment improves soil hydrological functioning in UK upland pastoral catchments

AbstractExtreme rainfall and flood events are predicted to increase in frequency and severity as a consequence of anthropogenic climate change. In UK upland areas, historical over‐grazing and associated soil compaction have further exacerbated peak flood levels and flash‐flood risk along many river catchments. As a result, the reinstatement of upland woodland is increasingly seen as a key component of an integrated suite of options forming part of natural flood management (NFM) associated with a 'public money for public goods' approach to European agriculture. Nevertheless, understanding the impact of native woodland establishment on upland soil hydrology remains relatively poor. We compare physical and hydrological properties from the surface soils of establishing woodland and grazed pasture across four flood vulnerable upland headwater catchments in Dartmoor National Park, SW England. We show upland native woodland establishment is a viable soil recovery option, with a doubling of soil saturated hydraulic conductivity, increased 'wetness threshold' and reduced surface soil compaction and bulk density within 15 years of establishment. Our study supports the establishment of native woodland as an effective tool to improve the hydrological functioning of soils in upland pastoral catchments and the provision of flash‐flood mitigation 'ecosystem services'. We caution, however, that land managers and policymakers must consider past and present management, soil type and catchment location when planning new NFM schemes if environmental benefits are to be maximised and 'public money for public goods, are to be commensurate with outcomes.

Interpreting event-based suspended sediment concentration and flow hysteresis patterns

This study analyses the hysteresis relationship between suspended sediment concentration (SSC) and flow (Q) during runoff events to investigate the effect of hydrological factors and catchment characteristics on suspended sediment dynamics. Continuous records of flow and suspended sediment concentration, proxied by turbidity, collected from 17 catchments across New Zealand with different size, land cover and erosion terrain characteristics were analysed for this purpose. We first classified the hysteresis patterns in terms of their shape and direction during events, then analysed hysteresis indices and related them to event hydrology and catchment characteristics. In total, 1553 events were analysed. The results indicate some clear differences between pasture- and forest-dominated catchments, with pasture-dominated catchments having event flow peaks generally lagging the peak of sediment concentration whereas in forest-dominated catchments the opposite occurred. Moreover, pasture catchments typically showed higher ratios of flow-weighted concentration on rising to falling stages of hydrographs compared with largely forested catchments. This shows that the sources of sediment respond faster during flood events in pasture-dominated catchments. A principal component and classification analysis of hydrological and sediment-related variables showed that the main variables controlling the hysteresis patterns within each catchment were flood total runoff and flood duration. These results are of immediate value to river management programmes and policies concerned with mitigating suspended sediment delivery to impacted waterways and to developing and testing event-scale catchment-based suspended sediment routing models. Future research could aim to reduce ambiguity in interpreting Q-SSC hysteresis driving factors by statistical analysis of larger datasets spanning more catchments and by using other information streams such as high spatio-temporal resolution sediment tracing.

The ability of detainment bunds to decrease surface runoff leaving pastoral catchments: Investigating a novel approach to agricultural stormwater management

Storm generated surface runoff is responsible for significant portions of the contaminants exported from grazed pastures that contribute to water quality impairment in inland and coastal waters. Detainment bunds (DBs) were investigated as a novel strategy to mitigate the losses of nutrients and sediment in surface runoff from pastures in the Lake Rotorua catchment, in New Zealand. A DB is a ∼1.5-2 m high earthen stormwater retention structure constructed on productive pastures in the flow path of targeted ephemeral streams. The current DB design protocol recommends a minimum pond volume of 120 m3 per hectare of contributing catchment. Bunds are capable of temporarily ponding up to 10,000 m3 of surface runoff, which can be rapidly drained by opening an outlet valve. This 12-month study of 2 DB sites with 55 ha and 20 ha subcatchments in the Lake Rotorua catchment, found that DBs effectively decreased annual discharge volumes by 31% and 43%. Decreased runoff discharges were the result of increased soil infiltration facilitated by increased stormwater residence times on well-drained soils in the ponding area. Furthermore, discharges from the DBs occurring after runoff generation in the catchment had ceased were likely to infiltrate the soils downstream of the DBs. Combining the in-pond and downstream infiltration, the DBs prevented 43% and 63% of the annual runoff generated in the targeted catchments from reaching downstream surface waters. The results of this study demonstrate that DBs constructed on sufficiently permeable soils reduce surface runoff volumes from pastures, and are thereby capable of decreasing contaminant loads delivered to receiving surface waters. As such, DBs are likely to be an effective strategy to add to the nutrient mitigation toolbox in the Lake Rotorua catchment, and in other pastoral locations where contaminants mobilised by surface runoff contribute to water quality degradation.

The stream hydrology response of converting a headwater pasture catchment to Pinus radiata plantation

ABSTRACT The planting of degraded land with tree plantations may be effective at improving land use sustainability and profitability but it can also have significant effects on stream hydrology. In this paired catchment study, we report the stream hydrological response to partial (62%) afforestation of a steep pastoral catchment in the western Waikato Region, North Island, New Zealand. We comprehensively analyse the hydrological regime changes over a 23-year period (including eight years before pine planting) with reference to a native-forested ‘control’ catchment. Our results show that afforestation has markedly affected stream hydrology. Seven years after planting, the total annual runoff was 380 mm lower than predicted for the catchment in pasture. Two phases of plantation thinning resulted in the difference between measured and predicted runoff reducing to only 129 mm. Peak flows reduced by ∼50% while total stormflow reduced by ∼30% – which we attribute to canopy interception attenuating and delaying water yield. The impact of plantation establishment on low flows is not so clear, although afforestation appears to have reduced low flows by ∼25%. This study provides information on the hydrological impact of afforestation within a hitherto poorly-represented New Zealand environment (i.e. high rainfall, sedimentary lithology-based, North Island hill country).

Understanding the effect of catchment characteristics on suspended sediment dynamics during flood events

AbstractHysteresis in the relationship between suspended sediment concentration and flow during run‐off events is commonly used to inform on sediment sources and hydrological pathways. Less attention, however, has been paid to comparing the water and sediment hydrographs, which provide a more direct appreciation of in‐event sediment dynamics and their relationship with the upstream catchment characteristics. The aim of this study is to better understand the catchment and hydrological controls on the phasing of water and sediment discharges during events and, in particular, to explore what controls sediment concentrations late on event recessions. Continuous records of flow and turbidity data (calibrated to suspended sediment concentration) were collected from 17 catchments across New Zealand for this purpose. Relationships between event sediment yield and peak flow showed, as anticipated, higher event sediment loads were generated in pasture compared with forested catchments and were also higher from catchments in more erodible terrain. One novel result was that these differences were greater during smaller, more frequent events, whereas the loads from larger flood events tended to converge between pasture and forest catchments. Another novel result was that event sediment load tends to be evenly split between rising and falling stages of the hydrograph in pasture catchments, but forested catchments yield more of their event loads on flood recessions, probably because of delayed erosion or more sediment sources remote from the channel network. Land cover, distance of the sediment sources from the monitoring site, and size of the catchments control sediment concentrations late on event recession. Pasture‐dominated and more erodible catchments show longer sediment recessions and therefore stay dirtier for longer time periods. In addition, the size of previous flood events appeared to control the extent of sediment exhaustion after the flood peaks in some catchments.

Siltation and radiocesium pollution of small lakes in different catchment types far from the Fukushima Daiichi nuclear power plant accident site

The Fukushima Daiichi nuclear power plant accident caused radioactive pollution in northeastern Honshu Island, Japan. This study examined the influence of snowmelt and rainfall on soil erosion processes and siltation of small lakes in Miyagi Prefecture (150 km northwest of the power plant). Two sets of slopes and lakes, respectively in pasture and forest catchments, were examined. Snowpack thickness, soil infiltration, surface runoff volume, soil and sediment physicochemical properties, Cs concentration of precipitation, meltwater, and rainwater, and lake siltation rates were determined. The total radioactive Cs content in precipitation was 0.7–7.4 BqˑL−1 and was below the Japanese standard (10 BqˑL−1). Total radioactive Cs was at the allowable level in water flowing down the pasture catchment slope (0.1–9.2 BqˑL−1) during snowmelt and rainfall, as well as in pasture (0.9–8.8 BqˑL−1) and forest (0.7–5.2 BqˑL−1) catchment lake water. There was no soil erosion (surface runoff) in the forest catchment. Soil losses in the pasture catchment were 23 due to rainfall and 9 kg ha−1 yr−1 following spring snowmelt. After snowmelt, a 0.5 and 0.2 mm thick layer of silt was deposited in pasture and forest catchment lakes, respectively, and 1.4 and 0.6 mm were deposited during the rainfall period. Average siltation rates were 1.9 and 0.8 mmˑyr−1 for pasture and forest catchment lakes, respectively. The upper layer of lake bottom sediments is represented mainly by silt fractions (2–50 μm), with high organic matter (4.0–5.7%) and radiocesium (1100–1600 kgˑha−1) contents.

Streams with Riparian Forest Buffers versus Impoundments Differ in Discharge and DOM Characteristics for Pasture Catchments in Southern Amazonia

Forest to pasture land use change following deforestation in Southern Amazonia can result in changes to stream water quality. However, some pasture streams have riparian forest buffers, while others are dammed for farm ponds. Stream corridor management can have differential effects on hydrology and dissolved organic matter (DOM) characteristics. We examined rainfall-runoff patterns and DOM characteristics in a pasture catchment with a forested riparian buffer, and an adjacent catchment with an impoundment. Total streamflow was 1.5 times higher with the riparian buffer, whereas stormflow represented 20% of total discharge for the dammed stream versus 13% with buffer. Stream corridor management was also the primary factor related to DOM characteristics. In the impounded catchment, DOM was found to be less structurally complex, with lower molecular weight compounds, a lesser degree of humification, and a larger proportion of protein-like DOM. In the catchment with a forested buffer, DOM was dominated by humic-like components, with fluorescence characteristics indicative of DOM derived from humified soil organic matter under native vegetation. Our results suggest that differences in stream corridor management can have important implications for carbon cycling in headwater pasture catchments, and that such changes may have the potential to influence water quality downstream in the Amazon basin.

Land cover effect on water discharge, matter losses and surface temperature: Results of 20 years monitoring in the Šumava Mts

Three small catchments (each c. 2 km2) with different land cover (drained pasture, wetland and spruce forest) located in the Šumava Mountains in the Czech Republic (altitude 780–1026 m) were monitored continuously for 20 years. The aim of the study was to evaluate the aging of different types of land cover landscape in terms of soil acidification (loss of alkalines). Average annual water discharge from the pasture catchment was c. 60 % of annual precipitation, but 38 % and 34 % of annual precipitation from the wetland and the forest, respectively. The relatively highest water runoff was from the pasture in all months. Daily runoffs during periods of heavy rain were 26 mm from the pasture, 16 mm from the wetland and 7 mm from the forested catchment. Water discharged from the drained pasture had two times higher electrical conductivity than water from the wetland and forest due to higher concentrations of bicarbonate, nitrate, calcium, magnesium, sodium and potassium. In the wetland and forest catchments, the amounts of nitrate, sulphate, chloride, TC, TN, TP and ammonium in the precipitated water were higher than that of the discharged with water (soluble matter losses). The amount of soluble matter discharged from the pasture catchment (matter losses) was in all cases higher than the amount in the precipitated water. On sunny days in the vegetation season, the pasture catchment had higher surface temperature than the wetland and forest catchments, which had higher evapotranspiration. Thermal pictures from satellites (e.g. Landsat) or taken by airborne thermal cameras can be used for identification of landscape areas of higher water outflow and higher matter losses.

Effects of land-use changes on structural characteristics of tropical high-altitude Andean headwater streams

Streams are usually susceptible to land-use change, mainly in the tropics due to high dynamic climatic conditions. Native forests have been converted for agricultural purposes with significantly impacts in streams. Nowadays, forests plantations are taking place of some degraded land and its influence in headwater streams are not well understood in tropical high-altitude streams. Thus, this study aims to assess effects of land-use changes from pasture to Eucalyptus plantations in Colombian Andean catchments on stream water conditions and structural characteristics of stream channels. The study was conducted in three catchments, one catchment covered by pasture, one catchment that was converted from pasture to Eucalyptus plantations in 1995 and one pristine catchment with native forest cover. Physical, chemical and biological conditions of stream water were assessed by measurements of water temperature, concentration of dissolved oxygen and chlorophyll-a content of epiphytic communities. The structural characteristics of stream channels were evaluated using a visual-based habitat assessment protocol from the United States Environmental Protection Agency. Principal Component Analysis (PCA) showed native forest and Eucalyptus plantations catchments associated with stream conservation characteristics and pasture catchment with overall degraded conditions. However, the Permutational multivariate analysis of variance (PERMANOVA) showed significant differences between all streams indicating that, despite the overall positive conservation aspects of native and Eucalyptus catchments, their still different from each other. Pasture catchment showed the highest values for temperature and chlorophyll-a, and the lowest values for dissolved oxygen and final score for structural characteristics. Therefore, our results demonstrated that the land-use change from pasture to Eucalyptus plantation improved the stream water conditions and the structural characteristics of the studied headwater streams. Additionally, we propose the use of the rapid bioassessment protocol coupled some stream water characteristics as a rapid and useful tool for detecting effects of land-use changes on high-altitude Andean streams.

Decline in New Zealand's freshwater fish fauna: effect of land use

The number of New Zealand’s freshwater fish listed as threatened has increased since 1992 when the first New Zealand threat classification system list was compiled. In this study, temporal and land cover-related trends were analysed for data on freshwater fish distribution, comprising more than 20000 records for the 47 years from January 1970 to January 2017 from the New Zealand Freshwater Fish Database. The analysis included individual species abundance and distribution trends, as well as an index of fish community integrity, namely the Index of Biotic Integrity (IBI). Of the 25 fish species that met the requirements for analysis to determine changes in the proportion of sites they occupied over time, 76% had negative trends (indicating declining occurrence). Of the 20 native species analysed for the proportion of sites occupied over time, 75% had negative trends; 65% of these were significant declines and more species were in decline at pasture sites than natural cover sites. The average IBI score also declined over the time period and, when analysed separately, the major land cover types revealed that the IBI declined at pasture catchment sites but not at sites with natural vegetation catchments.

Land Use‐Dependent Preferential Flow Paths Affect Hydrological Response of Steep Tropical Lowland Catchments With Saprolitic Soils

AbstractForested catchments in Central Panama can produce more base flow during the dry season compared to pasture catchments—the so‐called “forest sponge effect.” During rainfall events, peak storm runoff rates and storm runoff coefficients can be lower for forested catchments than pasture catchments, even when they have similar topographic characteristics, underlying geology, and soil texture. The internal mechanism of these differences in hydrological response due to land use is yet to be fully understood. A distributed model explicitly simulating preferential flow paths (PFPs), which is referred to as “PFPMod,” is used to explain the hydrological response caused by land use using data from three catchments with distinct land covers in Central Panama. Input parameters of forest and pasture land covers were identified using field observations and literature values. Multiple satisfactory objective criteria demonstrate that the two end‐member land cover parameter sets are adequate to explain the observed difference in dry‐season base flow and storm runoff coefficients. Field measurements of matrix infiltrability using soil cores and plot‐scale infiltration capacity enabled estimating the number of vertical macropores that fully penetrate the root zone. Model simulation results demonstrate that fast drainage through lateral PFPs in the early wet season and high flow in vertical PFPs to recharge deep groundwater in the late wet season contribute to the observed differences in peak storm runoff and the “forest sponge effect” during the dry season. This study provided insights to the mechanism by which reforestation may help to restore ecosystem services and water security in tropical settings.

Effects of conversion of native cerrado vegetation to pasture on soil hydro-physical properties, evapotranspiration and streamflow on the Amazonian agricultural frontier.

Understanding the impacts of land-use change on landscape-hydrological dynamics is one of the main challenges in the Northern Brazilian Cerrado biome, where the Amazon agricultural frontier is located. Motivated by the gap in literature assessing these impacts, we characterized the soil hydro-physical properties and quantified surface water fluxes from catchments under contrasting land-use in this region. We used data from field measurements in two headwater micro-catchments with similar physical characteristics and different land use, i.e. cerrado sensu stricto vegetation and pasture for extensive cattle ranching. We determined hydraulic and physical properties of the soils, applied ground-based remote sensing techniques to estimate evapotranspiration, and monitored streamflow from October 2012 to September 2014. Our results show significant differences in soil hydro-physical properties between the catchments, with greater bulk density and smaller total porosity in the pasture catchment. We found that evapotranspiration is smaller in the pasture (639 ± 31% mm yr-1) than in the cerrado catchment (1,004 ± 24% mm yr-1), and that streamflow from the pasture catchment is greater with runoff coefficients of 0.40 for the pasture and 0.27 for the cerrado catchment. Overall, our results confirm that conversion of cerrado vegetation to pasture causes soil hydro-physical properties deterioration, reduction in evapotranspiration reduction, and increased streamflow.

Quantifying the effects of Eucalyptus plantations and management on water resources at plot and catchment scales

AbstractOur aim was to quantify the effects of forest plantation and management (clear cut or 30% partial harvest) in relation to pasture, on catchment discharge in southeast Rio Grande do Sul state, Brazil. A paired‐catchment approach was implemented in two regions (Eldorado do Sul and São Gabriel municipalities) where discharge was measured for 4 years at three catchments in each region, two of which were predominantly eucalypt plantation (mainly Eucalyptus saligna, rotation of approximately 7–9 years) with native forest and grass in streamside zones. The third catchment was covered with grazed pasture. Weather, soils, canopy interception, groundwater level, tree growth, and leaf area index were also measured.The 3‐PG process‐based forest productivity model was adapted to predict spatial daily plantation and pasture water balance including precipitation interception, soil evaporation, transpiration, soil moisture, drainage, discharge, and monthly plantation growth. The TOPMODEL framework was used to simulate water pools and fluxes in the catchments.Discharge was higher under pasture than pre‐harvesting plantation and increased for 1–2 years after complete plantation harvest; this change was less pronounced in the catchments under partial harvest. The ratio of discharge to precipitation before harvesting varied from 7% to 13% in the eucalypt catchments and 28% to 29% under pasture. The ratio increases to 23–24% after total harvest, and to 17% after partial harvesting. The ratio under pasture also increases during this period (to 32–44%) owing to increased precipitation. The baseflow, in relation to total discharge, varied from 28% to 62% under Eucalyptus and from 38% to 43% in the pasture catchments. Hence, eucalypt plantations in these regions can be expected to influence discharge regimes when compared with pasture land use, and modelling suggests that partial harvesting would moderate the magnitude of discharge variation compared with a full catchment plantation harvesting. The model efficiency coefficient (Nash–Sutcliffe model efficiency coefficient) varied from 0.665 to 0.799 for the total period of the study. Simulation of alternative harvesting scenarios suggested that at least 20% of the catchment planted area must be harvested to increase discharge. This model could be a useful practical tool in various plantation forestry contexts around the world. Copyright © 2016 John Wiley & Sons, Ltd.

Water balance complexities in ephemeral catchments with different land uses: Insights from monitoring and distributed hydrologic modeling

AbstractAlthough ephemeral catchments are widespread in arid and semiarid climates, the relationship of their water balance with climate, geology, topography, and land cover is poorly known. Here we use 4 years (2011–2014) of rainfall, streamflow, and groundwater level measurements to estimate the water balance components in two adjacent ephemeral catchments in south‐eastern Australia, with one catchment planted with young eucalypts and the other dedicated to grazing pasture. To corroborate the interpretation of the observations, the physically based hydrological model CATHY was calibrated and validated against the data in the two catchments. The estimated water balances showed that despite a significant decline in groundwater level and greater evapotranspiration in the eucalypt catchment (104–119% of rainfall) compared with the pasture catchment (95–104% of rainfall), streamflow consistently accounted for 1–4% of rainfall in both catchments for the entire study period. Streamflow in the two catchments was mostly driven by the rainfall regime, particularly rainfall frequency (i.e., the number of rain days per year), while the downslope orientation of the plantation furrows also promoted runoff. With minimum calibration, the model was able to adequately reproduce the periods of flow in both catchments in all years. Although streamflow and groundwater levels were better reproduced in the pasture than in the plantation, model‐computed water balance terms confirmed the estimates from the observations in both catchments. Overall, the interplay of climate, topography, and geology seems to overshadow the effect of land use in the study catchments, indicating that the management of ephemeral catchments remains highly challenging.

Spatial and temporal variations in nitrogen export from a New Zealand pastoral catchment revealed by stream water nitrate isotopic composition

AbstractViable indicators of nitrogen (N) attenuation at the catchment scale are needed in order to sustainably manage global agricultural intensification. We hypothesized that the dominance of a single land use (pasture production) and strong ground‐to‐surface water connectivity would combine to create a system in which surface water nitrate isotopes (δ15N and δ18O of ) could be used to monitor variations in catchment‐scale attenuation. Nitrate isotopes were measured monthly over a 2 year period in four reaches along a spring‐fed, gaining stream (mean ‐N of 6 mg L−1) in Canterbury, New Zealand. The stream water pool indicated that the highest degree of denitrification occurred in the shallow upper reaches. Moving downstream through increasingly sandy soils, the isotopic signature of denitrification became progressively weaker. The lowest reaches fell into the expected range for produced from the nitrification of pasture N sources (urine and fertilizers), implying that the attenuation capacity of the groundwater and riparian systems was lower than the rate of N inputs. After excluding months affected by effluent spills or extreme weather (n = 4), variations in the degree of denitrification over stream distance were combined with the measured discharge to estimate N attenuation over time in the subcatchment. Attenuation was highly responsive to rainfall: 93% of calculated attenuation (20 kg ‐N ha−1 yr−1) occurred within 48 h of rainfall. These findings demonstrate the potential for detailed stable isotope data to provide integrative measures of catchment loss pathways.

The Brigalow Catchment Study: IV. Clearing brigalow (Acacia harpophylla) for cropping or grazing increases peak runoff rate

In Queensland, Australia, large tracts of native vegetation have been cleared for agriculture, resulting in substantial hydrological changes in the landscape. Australia’s longest-running paired catchment study, the Brigalow Catchment Study (BCS), was established in 1965 to monitor hydrological changes associated with land development, particularly that of the 1960s Land Development Fitzroy Basin Scheme. The BCS has unequivocally shown that developing brigalow (Acacia harpophylla) for cropping or for grazing doubles runoff volume. However, to date little research had been undertaken to quantify the changes in peak runoff rate when brigalow is cleared for cropping or grazing. The present study compared peak runoff rates from three brigalow catchments, two of which were subsequently cleared for cropping and pasture. Prior to land development, average peak runoff rates from the three brigalow scrub catchments were 3.2, 5 and 2mmh–1 for catchments 1 to 3 respectively. After development, these rates increased to 6.6mmh–1 from the brigalow scrub control catchment (catchment 1), 8.3mmh–1 from the cropping catchment (catchment 2) and 5.6mmh–1 from the pasture catchment (catchment 3). Peak runoff rate increased significantly from both the cropping and pasture catchments after adjusting for the underlying variation in peak runoff rate due to climatic variation between the pre- and post-development periods. The average peak runoff rate increased by 5.4mmh–1 (96%) for the cropping catchment and by 2.6mmh–1 (47%) for the pasture catchment. Increases in peak runoff rate were most prevalent in smaller events with an average recurrence interval of less than 2 years under cropping and 4 years under pasture.