Summer Rainfall Events Research Articles

Contrasting physiological responsiveness of establishing trees and a C4 grass to rainfall events, intensified summer drought, and warming in oak savanna

AbstractClimate warming and drought may alter tree establishment in savannas through differential responses of tree seedlings and grass to intermittent rainfall events. We investigated leaf gas exchange responses of dominant post oak savanna tree (Quercus stellata and Juniperus virginiana) and grass (Schizachyrium scoparium, C4 grass) species to summer rainfall events under an ambient and intensified summer drought scenario in factorial combination with warming (ambient, +1.5 °C) in both monoculture and tree‐grass mixtures. The three species differed in drought resistance and response of leaf gas exchange to rainfall events throughout the summer. S. scoparium experienced the greatest decrease in Aarea (−56% and −66% under normal and intensified drought, respectively) over the summer, followed by Q. stellata (−44%, −64%), while J. virginiana showed increased Aarea under normal drought (+13%) and a small decrease in Aarea when exposed to intensified summer drought (−10%). Following individual rainfall events, mean increases in Aarea were 90% for S. scoparium, 26% for J. virginiana and 22% for Q. stellata. The responsiveness of Aarea of S. scoparium to rainfall events initially increased with the onset of drought, but decreased dramatically as summer drought progressed. For Q. stellata, Aarea recovery decreased as drought progressed and with warming. In contrast, J. virginiana showed minimal fluctuations in Aarea following rainfall events, in spite of declining water potential, and warming enhanced recovery. J. virginiana will likely gain an advantage over Q. stellata during establishment under future climatic scenarios. Additionally, the competitive advantage of C4 grasses may be reduced relative to trees, because grasses will likely exist below a critical water stress threshold more often in a warmer, drier climate. Recognition of unique species responses to critical global change drivers in the presence of competition will improve predictions of grass–tree interactions and tree establishment in savannas in response to climate change.

Annual methane uptake by typical semiarid steppe in Inner Mongolia

Steppe ecosystems cover approximately 10% of the global land surface. Recent measurements have shown that steppe soils function as a significant sink for atmospheric methane (CH4). However, precise quantification of the annual CH4 uptake by steppe is challenged by infrequent measurements of exchange rates, which often only cover the growing season. In order to understand the annual dynamics and magnitude of CH4 exchange, especially contribution of nongrowing season to the cumulative annual CH4 exchange, we conducted year‐round CH4 flux measurements at high temporal resolution at two adjacent steppe sites. One was ungrazed and fenced since 1999 (UG99) and the other was grazed during the winter (WG01). The measurements were supplemented with observations of CH4 concentrations in the soil profile. Sites were located in typical Leymus chinensis steppe in Inner Mongolia, China. The results show that the typical semiarid steppe functioned exclusively as a sink for atmospheric CH4 throughout the entire year. Even during the spring soil thawing, a period with high water content in the top soil, CH4 uptake was dominant. The seasonality of CH4 uptake displayed a strong dependency on the seasonal variation in soil temperature. Soil moisture increased in importance when temperature was not the limiting factor. For example, CH4 rates decreased sharply following summer rainfall events. The annual CH4 uptake by the ungrazed UG99 and the winter‐grazed WG01 sites was 3.7 and 2.1 kg C ha−1, respectively. The contribution of the nongrowing season (October–April) to the cumulative annual CH4 uptake was approximately 30% (25%–36%). Additionally, our data suggest that winter grazing significantly alters the capacity of steppe soils for CH4 uptake. However, more measurements at paired ungrazed/grazed sites are needed to assess how grazing might affect the CH4 uptake capacity of steppe soils at a larger regional or global scale.

Are intraseasonal summer rainfall events micro monsoon onsets over the western edge of the South-Asian monsoon?

A thermodynamic structure leading the active phase (AP) of the Western Edge of the South-Asian Monsoon (WESAM) is investigated. The APs seems to have significant contribution in the mean seasonal rainfall in the region. A few days before APs, the upper level warm anomaly appears over the north Hindu Kush-Himalaya (HKH) region and it is reinforced by surface heating yielding the column average warming. The height anomalies are baroclinic with the low-level anticyclone being located at the east of warming. The low-level anticyclone causes the moisture convergence at the core WESAM region. As the region keeps warming, the height anomalies and associated low-level convergence become stronger. The AP starts when the low-level moisture convergence is strong enough to overcome the preexisting stable atmospheric condition due to the upper level warming. The proposed mechanism of APs has some resemblance with large scale south Asian monsoon onset, whereas conventional south Asian monsoon intraseasonal oscillations (ISOs) do not show clear relationship with APs of WESAM.

Carbon dioxide, methane, and nitrous oxide exchanges in an age‐sequence of temperate pine forests

AbstractWe investigated soil carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) exchanges in an age‐sequence (4, 17, 32, 67 years old) of eastern white pine (Pinus strobus L.) forests in southern Ontario, Canada, for the period of mid‐April to mid‐December in 2006 and 2007. For both CH4 and N2O, we observed uptake and emission ranging from −160 to 245 μg CH4 m−2 h−1 and −52 to 21 μg N2O m−2 h−1, respectively (negative values indicate uptake). Mean fluxes from mid‐April to mid‐December across the 4, 17, 32, 67 years old stands were similar for CO2 fluxes (259, 246, 220, and 250 mg CO2 m−2 h−1, respectively), without pattern for N2O fluxes (−3.7, 1.5, −2.2, and −7.6 μg N2O m−2 h−1, respectively), whereas the uptake rates of CH4 increased with stand age (6.4, −7.9, −10.8, and −23.3 μg CH4 m−2 h−1, respectively). For the same period, the combined contribution of CH4 and N2O exchanges to the global warming potential (GWP) calculated from net ecosystem exchange of CO2 and aggregated soil exchanges of CH4 and N2O was on average 4%, <1%, <1%, and 2% for the 4, 17, 32, 67 years old stand, respectively. Soil CO2 fluxes correlated positively with soil temperature but had no relationship with soil moisture. We found no control of soil temperature or soil moisture on CH4 and N2O fluxes, but CH4 emission was observed following summer rainfall events. LFH layer removal reduced CO2 emissions by 43%, increased CH4 uptake during dry and warm soil conditions by more than twofold, but did not affect N2O flux. We suggest that significant alternating sink and source potentials for both CH4 and N2O may occur in N‐ and soil water‐limited forest ecosystems, which constitute a large portion of forest cover in temperate areas.

Conventional and Conservation Tillage: Influence on Seasonal Runoff, Sediment, and Nutrient Losses in the Canadian Prairies

Conservation tillage has been widely promoted to reduce sediment and nutrient transport from agricultural fields. However, the effect of conservation tillage on sediment and nutrient export in snowmelt-dominated climates is not well known. Therefore, a long-term paired watershed study was used to compare sediment and nutrient losses from a conventional and a conservation tillage watershed in the Northern Great Plains region of western Canada. During the treatment period, dissolved nutrient concentrations were typically greater during spring snowmelt than during summer rainfall events, whereas concentrations of sediment and particulate nutrients were greatest during rainfall events. However, because total runoff was dominated by snowmelt, most sediment and nutrient export occurred during snowmelt. Overall, conservation tillage reduced the export of sediment in runoff water by 65%. Similarly, concentrations and export of nitrogen were reduced by 41 and 68%, respectively, relative to conventional tillage. After conversion to conservation tillage, concentrations and exports of phosphorus (P) increased by 42 and 12%, respectively, with soluble P accounting for the majority of the exported P, especially during snowmelt. Our results suggest that management practices designed to improve water quality by reducing sediment and sediment-bound nutrient export from agricultural fields and watersheds can be less effective in cold, dry regions where nutrient export is primarily snowmelt driven and in the dissolved form. In these situations, it may be more appropriate to implement management practices that reduce the accumulation of nutrients in crop residues and the surface soil.

Incorporating landscape depressions and tile drainages of a northern German lowland catchment into a semi‐distributed model

AbstractHydrological models need to be adapted to specific hydrological characteristics of the catchment in which they are applied. In the lowland region of northern Germany, tile drains and depressions are prominent features of the landscape though are often neglected in hydrological modelling on the catchment scale. It is shown how these lowland features can be implemented into the Soil and Water Assessment Tool (SWAT). For obtaining the necessary input data, results from a GIS method to derive the location of artificial drainage areas have been used. Another GIS method has been developed to evaluate the spatial distribution and characteristics of landscape depressions. In the study catchment, 31% of the watershed area is artificially drained, which heavily influences groundwater processes. Landscape depressions are common over the 50‐km2 study area and have considerable retention potential with an estimated surface area of 582 ha. It was the scope of this work to evaluate the extent by which these two processes affect model performance. Accordingly, three hypotheses have been formulated and tested through a stepwise incorporation of drainage and depression processes into an auto calibrated default setup: (1) integration of artificial drainage alone; (2) integration of depressions alone and (3) integration of both processes combined. The results show a strong improvement of model performance for including artificial drainage while the depression setup only induces a slight improvement. The incorporation of the two landscape characteristics combined led to an overall enhancement of model performance and the strongest improvement in r2, root mean square error (RMSE) and Nash–Sutcliffe efficiency (NSE) of all setups. In particular, summer rainfall events with high intensity, winter flows and the hydrograph's recession limbs are depicted more realistically. Copyright © 2010 John Wiley & Sons, Ltd.

Soil nitrous oxide and methane fluxes are low from a bioenergy crop (canola) grown in a semi‐arid climate

AbstractUnderstanding nitrous oxide (N2O) and methane (CH4) fluxes from agricultural soils in semi‐arid climates is necessary to fully assess greenhouse gas emissions from bioenergy cropping systems, and to improve our knowledge of global terrestrial gaseous exchange. Canola is grown globally as a feedstock for biodiesel production, however, resulting soil greenhouse gas fluxes are rarely reported for semi‐arid climates. We measured soil N2O and CH4 fluxes from a rain‐fed canola crop in a semi‐arid region of south‐western Australia for 1 year on a subdaily basis. The site included N fertilized (75 kg N ha−1 yr−1) and nonfertilized plots. Daily N2O fluxes were low (−1.5 to 4.7 g N2O‐N ha−1 day−1) and culminated in an annual loss of 128 g N2O‐N ha−1 (standard error, 12 g N2O‐N ha−1) from N fertilized soil and 80 g N2O‐N ha−1 (standard error, 11 g N2O‐N ha−1) from nonfertilized soil. Daily CH4 fluxes were also low (−10.3 to 11.9 g CH4‐C ha−1 day−1), and did not differ with treatments, with an average annual net emission of 6.7 g CH4–C ha−1 (standard error, 20 g CH4–C ha−1). Greatest daily N2O fluxes occurred when the soil was fallow, and following a series of summer rainfall events. Summer rainfall increased soil water contents and available N, and occurred when soil temperatures were >25 °C, and when there was no active plant growth to compete with soil microorganisms for mineralized N; conditions known to promote N2O production. The proportion of N fertilizer emitted as N2O, after correction for emissions from the no N fertilizer treatment, was 0.06%; 17 times lower than IPCC default value for the application of synthetic N fertilizers to land (1.0%). Soil greenhouse gas fluxes from bioenergy crop production in semi‐arid regions are likely to have less influence on the net global warming potential of biofuel production than in temperate climates.

Radiocarbon chronology of Holocene colluvial (debris-flow) events at Sletthamn, Jotunheimen, southern Norway: a window on the changing frequency of extreme climatic events and their landscape impact

The history of colluvial events over the last 8500 years is reconstructed in three Norwegian alpine slope-foot mires fed by three independent debris-flow systems. Chronologies for each site are constructed based on 155 radiocarbon-dated peat samples. At the multimillennial scale, debris-flow activity was greatest during the transition from the Holocene Thermal Maximum to the late Holocene ( c. 4300—2800 cal. BP) when debris-flow events occurred with a frequency of 1 in 33 years: two peaks in activity, characterized by 1 event in 14 years and 1 event in 25 years, were reached at the beginning and end of this interval, respectively. Least activity occurred during the Holocene Thermal Maximum from c. 8000 to 7100 cal. BP, with one event in 900 years. Eight distinct century- to millennial-scale phases of high debris-flow frequency (>3 events per 100 years) are identified at c. 8300—8000, 7100—7000, 4300—3700, 3200—2800, 2200—1900, 1500—1200, 800—700 and 300—0 cal. BP. Similarities in the records from the three sites suggest underlying climatic variations linked to the frequency of intense summer and autumn rainfall events (the primary meteorological trigger of slope failure in the source areas). Differences between the records reflect local site sensitivity to the initiation of debris-flow activity and the triggering of each subsequent debris-flow event. It is also inferred that each debris-flow system passes through multimillennial stages of at first increasing and later decreasing sensitivity as its source area expands. The climatic signals in the debris-flow record appear to differ in various respects from those derived from other precipitation-sensitive proxies from southern Norway (river floods, snow avalanches and glacier variations). Debris-flow records provide, therefore, complementary information relating to extreme climatic events and demonstrate instabilities in the Holocene landscape but provide little or no support for the concept of an increasing landscape impact of debris flows in response to global warming.

Nitrous oxide emissions from an apple orchard soil in the semiarid Loess Plateau of China

Nitrous oxide (N2O) fluxes from an apple orchard soil in the semiarid Loess Plateau of China were measured using static chambers from September 2007 to September 2008. In this study, three sites were selected at distance of 2.5 m (D 2.5), 1.5 m (D 1.5), and 0.5 m (D 0.5) from the apple tree row. Nitrous oxide fluxes followed seasonal pattern, with high N2O emission rates occurring in the hot-humid summer and low rates in the cold-dry winter. Pulses of N2O emissions occurred after nitrogen fertilizer application, summer rainfall events, and during freeze-thaw cycles. Annual average N2O emission rates were the highest at D 0.5 site (48.2 ± 39.9 µg N2O m−2 h−1), the lowest at D 2.5 site (31.9 ± 18.2 µg N2O m−2 h−1), and intermediate at D1.5 site (36.8 ± 32.2 µg N2O m−2 h−1), suggesting that N2O emissions from the apple orchard soil increased when the chamber location was closer to the apple tree row. This may be due to the fertilization close to roots in hot and humid season. Over one third (37.1%) of the annual N2O emission occurred in the summer. Annual N2O emissions from the apple orchard soil averaged to 3.22 kg N2O ha−1 year−1. Annual emission factor of the apple orchard from the applied fertilizer (uncorrected for background emission) was 0.658%. This value was nearly a half (53%) of the default value provided by the Intergovernmental Panel on Climate Change for the application of synthetic fertilizers to cropland (1.25%). Therefore, the amount of N2O emissions from the semiarid apple orchard soil could be largely overestimated if no regional-specific factor is used.

Large-scale atmospheric conditions associated with heavy rainfall episodes in Southeast Brazil

Heavy rainfall events in austral summer are responsible for almost all the natural disasters in Southeast Brazil. They are mostly associated with two types of atmospheric perturbations: Cold Front (53%) and the South Atlantic Convergence Zone (47%). The important question of what synoptic characteristics distinguish a heavy rainfall event (HRE) from a normal rainfall event (NRE) is addressed in this study. Here, the evolutions of such characteristics are identified through the anomalies with respect to climatology of the composite fields of atmospheric variables. The anomalies associated with HRE are significantly more intense than those associated with NRE in all fundamental atmospheric variables such as outgoing long-wave radiation, sea-level pressure, 500-hPa geopotential, lower and upper tropospheric winds. The moisture flux convergence over Southeast Brazil in the HRE composites is 60% larger than in the NRE composites. The energetics calculations for the HRE that occurred in the beginning of February 1988 strongly suggest that the barotropic instability played an important role in the intensification of the perturbation. These results, especially the intensities of the wind, pressure anomalies, and the moisture convergence are useful for the meteorologists of the Southeast Brazil for forecasting heavy precipitation.

The apparent demise of the Asian date mussel Musculista senhousia in Western Australia: or using acts of god as an eradication tool

Musculista senhousia (Benson in Cantor, 1842) is a small thin-shelled mytilid inhabiting intertidal and shallow subtidal sediments. Prior to 2000, M. senhousia was widespread in the Swan River estuary, Perth, Western Australia. In 2000 the Perth region experienced its largest recorded summer rainfall event, with over 270 GL entering the estuary. The rainfall formed a freshwater lens over much of the river and a subsequent toxic algal bloom of the cyanobacterium Microcystis aeruginosa. M. aeruginosa produces a toxin microcystin, known to have adverse effects upon molluscs. Surveys to assess the status of M. senhousia in the estuary in 2007 failed to find any individuals. We suggest the apparent death of M. senhousia from the estuary may be attributable to a combination of high seasonal variability in the mussel’s populations, high summer rainfall reducing salinity, and a toxic algal bloom that eliminated any remaining M. senhousia individuals.

Three approaches to estimate inorganic nitrogen loading under varying climatic conditions from a headwater catchment in Finland

Inorganic nitrogen loading was simulated using two dynamic catchment scale models, Integrated Nutrients in Catchments–Nitrogen (INCA-N) and the Generalized Watershed Loading Functions (GWLF). The simulated N loading was compared to a standard method to calculate annual loading using measured discharge and discharge-weighted concentrations. The main aim of the study was to compare these three estimation approaches with regards to their performance in hydrologically variable years in a small headwater catchment in southern Finland. Inter-annual variability of INCA-N and GWLF was compared with measured inorganic N concentrations at the catchment outlet. In years where snow melt dominates the annual discharge pattern all methods gave concurrent annual loading estimates. However, the loading estimates differ between the studied methods in years where large rainfall events in late summer or autumn dominate the annual discharge pattern, or when the model was not able to reproduce the spring discharge maximum properly. The results suggest that both models can be useful tools in estimating dissolved inorganic nitrogen loading from a catchment under changing climate conditions, providing that the key influencing driver, hydrology, is well captured.

Phosphorus dynamics in the ditch system of a restored peat wetland

The restoration of wetlands as bird habitats often involves the maintenance of a fluctuating water regime by careful, localised ditch water management using pumps and sluices. However, there is evidence in the literature to suggest that alternate flood/drainage cycles can accelerate nutrient cycling and transport within the soil and, therefore, pose a threat to water quality through the process of eutrophication. This study focused on the dynamics and losses of soil P in a recently re-wetted, eutrophic fen peat developed on alluvium in south west England. During the 2-year study (2001 and 2002), soil water tensiometry revealed that the field water table (fluctuating annually between +20 and −60 cm relative to ground level) was extensively influenced across most of the 8.4 ha field site by the management of the adjacent ditch water levels. This conservation-led, prescribed water balance was facilitated by the high hydraulic conductivity (1.1 × 10 −5 m s −1) of the lower (70–140 cm), degraded layer of peat. However, only during a 7-day period of water table drawdown by intermittent pump drainage, approximately 45 g ha −1 of dissolved reactive P (DRP) entered the pumped ditch from the field via this degraded layer. Summer rainfall events >35 mm d −1 also coincided with significant peaks in ditch water P concentration (up to 200 μg L −1 DRP). Even larger peaks (up to 700 μg L −1 DRP) occurred with the annual onset of autumn reflooding. These episodic P loss events pose a serious potential threat to biological water quality.

Impacts of urban expansion and future green planting on summer precipitation in the Beijing metropolitan area

In this study, an analysis of long‐term rainfall data reveals that the rapid urban expansion in Beijing since 1981 is statistically correlated to summer rainfall reduction in the northeast areas of Beijing from 1981 to 2005. This coincides with the period in which the shortage of water in the Beijing area has become a serious factor for sustainable economic development. Meanwhile, an analysis of the aerosol optical depth (AOD) from the Total Ozone Mapping Spectrometer spanning the years from 1980 to 2001 shows that there is no clear secular trend in summer AOD in Beijing. With the particular purpose of further understanding the effects of urban expansion on summer rainfall and the potential measures to mitigate such effects, a mesoscale weather/land‐surface/urban–coupled model along with different urban land‐use change scenarios are used to conduct numerical simulations for two selected heavy summer rainfall events with different, but representative, summer weather patterns in Beijing. Results show that urban expansion can produce less evaporation, higher surface temperatures, larger sensible heat fluxes, and a deeper boundary layer. This leads to less water vapor, more mixing of water vapor in the boundary layer, and hence less (more) convective available potential energy (convective inhibition energy). The combination of these factors induced by expanding urban surfaces is helpful in reducing precipitation for the Beijing area in general and, in particular, for the Miyun reservoir area (the major source for the local water supply). Increasing green vegetation coverage in the Beijing area would produce more rainfall, and model results show that planting grass seems more effective than planting trees. For the same vegetation, the rainfall difference from simulations using two green‐planting layouts (annular and cuneiform) is small.

Feed gaps in mixed-farming systems: insights from the Grain & Graze program

A central concern of the Grain & Graze research, development and extension program has been improving the management of the feedbase on mixed farms through addressing ‘feed gaps’ – times of year during which the supply of forage is insufficient to meet livestock demand. In this review, we use the available data on pasture growth and quality, supplemented by modelling results, to describe the characteristic timing of feed gaps across the Australian cereal-livestock zone. Feedbase interventions studied during the Grain & Graze program have mainly addressed the supply side of the feed balance equation. We review these studies, paying particular attention to the time scale of the variability in the feed balance that each intervention is intended to address. We conclude that grazing of cereals (either dual-purpose or forage crops) is the most promising means of alleviating winter feed gaps in regions where they are important. Reducing feed gaps in summer by relying on unpredictable summer rainfall events will increase year-to-year variability in forage production and will therefore require more flexible livestock management systems to exploit it. The use of forage shrubs offers a practical tool for increasing the predictability of summer and autumn feed supply, but given their moderate capacity for providing additional metabolisable energy it remains important to carefully manage livestock over autumn and to manage the herbaceous inter-row pasture. Feed gaps mainly arise from an interaction between biology and economics. We find, however, that the options studied in the Grain & Graze program for addressing feed gaps require either greater complexity in pasture and grazing management or more opportunistic livestock trading; they therefore come at a cost to the manager’s limited decision-making time. Times with feed gaps are also times when particular natural resource management risks (especially erosion) need to be managed. Supply-side interventions to relieve feed gaps will generally use more soil water, which will often have positive effects on natural resource management outcomes.

Rainfall trends in the KwaZulu‐Natal Drakensberg region of South Africa during the twentieth century

AbstractThis study assesses long‐term rainfall records in the KwaZulu‐Natal Drakensberg, South Africa's most valuable source of surface runoff. Records from 11 stations covering the Drakensberg region in South Africa indicate that no statistically significant trend in interannual variability existed during the last half of the twentieth century. Annual rainfall in the Drakensberg is highly seasonal, and analysis of monthly rainfall indicates an increase in the variability of the distribution of monthly rainfall and the strengthening of rainfall seasonality in the Drakensberg through a significant decrease in autumn rainfall. The El Niño/Southern Oscillation (ENSO) influences summer rainfall variability of the KwaZulu‐Natal Drakensberg, and a strong correlation also exists between summer rainfall and the Southern Oscillation Index (SOI) for preceding periods. This correlation between summer rainfall and ENSO events suggests that changes in the frequency and intensity of the ENSO should affect the rainfall in the Drakensberg. The lagged correlation between summer rainfall in the Drakensberg and the SOI could be used as an indicator for seasonal forecasting. Copyright © 2008 Royal Meteorological Society

Influence of a Large Late Summer Precipitation Event on Food Limitation and Grasshopper Population Dynamics in a Northern Great Plains Grassland

The complex interplay between grasshoppers, weather conditions, and plants that cause fluctuations in grasshopper populations remains poorly understood, and little is known about the ecological processes that generate grasshopper outbreaks. Grasshopper populations respond to interacting extrinsic and intrinsic factors, with yearly and decadal weather patterns and the timing of precipitation all potentially important. The effects of initial and increasing grasshopper densities on grasshopper survival and reproductive correlates were examined at a northern mixed-grass prairie site through manipulations of grasshopper densities inside 10-m2 cages. High-quality grass growth occurred after a 9.1-cm mid-August rain. Reduced proportional survival was apparent in the two higher density treatments before the rain, indicative of food-limited density-dependent mortality. However, the large late summer rainfall event mediated the effects of exploitative competition on demographic characteristics because of the high-quality vegetation growth. This led to weaker effects of food limitation on survival and reproduction at the end of the experiment. The results indicate a direct link between weather variation, resource quality and grasshopper population dynamics led to a severe grasshopper outbreak and show that infrequent large precipitation events can have significant effects on population dynamics. Additional research is needed to examine the importance of infrequent large precipitation events on grasshopper population dynamics in grassland ecosystems.

Water balance of centre pivot irrigated pasture in northern Victoria, Australia

The irrigated dairy industry in Australia depends on pasture as a low-cost source of fodder for milk production. The industry is under increasing pressure to use limited water resources more efficiently. Pasture is commonly irrigated using border-check but there is growing interest amongst dairy irrigators to explore the potential for overhead sprinklers to save water and/or increase productivity. This paper reports on a detailed water balance study that evaluated the effectiveness of centre pivot irrigation for pasture production. The study was conducted between 2004/2005 and 2005/2006 on a commercial dairy farm in the Shepparton Irrigation Region in northern Victoria. More than 90% of supplied water (irrigation plus rainfall) was utilized for pasture growth. Deep drainage of respectively 90 and 93 mm was recorded for the two observation seasons. During the 2004/2005 season, deep drainage resulted from large unseasonal summer rainfall events. Over the 2005/2006 season, deep drainage resulted from excess irrigation. The cumulative pasture dry matter (DM) production was 15.5 and 11.3 tonnes DM ha −1 for the two irrigation seasons, with an agronomic water use efficiency (WUE) of 16 and 12 kg DM ha −1 mm −1 respectively. The farmer's intuitive irrigation scheduling was found to be very effective; the pattern of irrigation application closely matched measured pasture water use, prevented water stress and resulted in high irrigation efficiency.

Moisture pulses, trace gas emissions and soil C and N in cheatgrass and native grass-dominated sagebrush-steppe in Wyoming, USA

Effects of large-scale weed invasion on the nature and magnitude of moisture-pulse-driven soil processes in semiarid ecosystems are not clearly understood. The objective of this study was to monitor carbon dioxide (CO2) and nitrous oxide (N2O) emissions and changes in soil carbon (C) and nitrogen (N) following the application of a water pulse in Wyoming big sagebrush (Artemisia tridentata ssp. wyomingensis) communities dominated by the exotic annual grass cheatgrass (Bromus tectorum) and by the native perennial grass western wheatgrass (Pascopyrum smithii). Sampling locations were established in shrub interspaces dominated by B. tectorum and P. smithi and beneath shrub canopies adjacent to interspaces dominated by B. tectorum and P. smithi, where no grass was present. Soils were classified as fine-loamy, mixed, Borollic Haplargids. Soil samples (0–10cm) and air samples were collected at 0, 4, 8, 24, 49, 72, and 216h following additions of 25.4mm of water. Soil samples were analyzed for dissolved organic carbon (DOC), microbial biomass carbon (MBC), extractable ammonia (NH4+), extractable nitrate (NO3−), and dissolved organic nitrogen (DON). Grass species induced differences in soil nitrification, N2O and CO2 emissions, and the quantity and timing of labile C available to microbial populations responding to increased moisture availability. In the first 8-h phase after wetting P. smithii soils had the greatest CO2 emissions compared to other soils but B. tectorum soils had the greatest N2O emissions and the greatest increases in CO2 emissions relative to before wetting. Microbial biomass C in B. tectorum interspace soils increased rapidly but the response was short-lived despite sufficient water availability. After the first 8h of soil response to wetting, the observed MBC declines in B. tectorum interspace coincided with disproportional DOC and DON concentration increases. Similar DOC and DON increases were also observed in B. tectorum soils beneath shrub canopy. In contrast, DOC and DON concentrations in P. smithii soils remained unaffected by soil wetting and small MBC increases observed during the first 8-h phase did not decline as rapidly as in B. tectorum interspace soils. In conclusion, summer drying-wetting cycles that occur frequently in areas invaded by B. tectorum can accelerate rates of nitrification and C mineralization, and contribute significantly to trace gas emissions from sagebrush-steppe grasslands. With frequent summer rainfall events, the negative consequences B. tectorum presence in the ecosystem can be significant.

Solifluction processes in an area of seasonal ground freezing, Dovrefjell, Norway

AbstractContinuous monitoring of soil temperatures, frost heave, thaw consolidation, pore water pressures and downslope soil movements are reported from a turf‐banked solifluction lobe at Steinhøi, Dovrefjell, Norway from August 2002 to August 2006. Mean annual air temperatures over the monitored period were slightly below 0°C, but mean annual ground surface temperatures were around 2°C warmer, due to the insulating effects of snow cover. Seasonal frost penetration was highly dependent on snow thickness, and at the monitoring location varied from 30–38 cm over the four years. The shallow annual frost penetration suggests that the site may be close to the limit of active solifluction in this area. Surface solifluction rates over the period 2002–06 ranged from 0.5 cm yr−1 at the rear of the lobe tread to 1.6 cm yr−1 just behind the lobe front, with corresponding soil transport rates of 6 cm3 cm−1 yr−1 and 46 cm3 cm−1 yr−1. Pore water pressure measurements indicated seepage of snowmelt beneath seasonally frozen soil in spring with artesian pressures beneath the confining frozen layer. Soil thawing was associated with surface settlement and downslope soil displacements, but following clearance of the frozen ground, later soil surface settlement was accompanied by retrograde movement. Summer rainfall events caused brief increases in pore pressure, but no further soil movement. Surface displacements exceeded maximum potential frost creep values and it is concluded that gelifluction was an important component of slow near‐surface mass movements at this site. Temporal and spatial variations in solifluction rates across the area are likely to be considerable and strongly influenced by snow distribution. Copyright © 2008 John Wiley & Sons, Ltd.