Controls on evapotranspiration from jack pine forests in the Boreal Plains Ecozone

The exchanges of water, energy and carbon between the land surface and the atmosphere are tightly coupled, so that errors in simulating evapotranspiration lead to errors in simulating both the water and carbon balances. Areas with seasonally frozen soils present a particular challenge due to the snowmelt-dominated hydrology and the impact of soil freezing on the soil hydraulic properties and plant root water uptake. Land surface schemes that have been applied in high latitudes often have reported problems with simulating the snowpack and runoff. Models applied at the Boreal Ecosystem Research and Monitoring Sites in central Saskatchewan have consistently over-predicted evapotranspiration as compared with flux tower estimates. We assessed the performance of two Canadian land surface schemes (CLASS and CLASS-CTEM) for simulating point-scale evapotranspiration at an instrumented jack pine sandy upland site in the southern edge of the boreal forest in Saskatchewan, Canada. Consistent with past reported results, these models over-predicted evapotranspiration, as compared with flux tower observations, but only in the spring period. Looking systematically at soil properties and vegetation characteristics, we found that the dominant control on evapotranspiration within these models was the canopy conductance. However, the problem of excessive spring ET could not be solved satisfactorily by changing the soil or vegetation parameters. The model overestimation of spring ET coincided with the overestimation of spring soil liquid water content. Improved algorithms for the infiltration of snowmelt into frozen soils and plant-water uptake during the snowmelt and soil thaw periods may be key to addressing the biases in spring ET.     

Lateral flow thresholds for aspen forested hillslopes on the Western Boreal Plain,Alberta, Canada

To predict the long‐term sustainability of water resources on the Boreal Plain region of northern Alberta, it is critical to understand when hillslopes generate runoff and connect with surface waters. The sub‐humid climate (P ≤ ET) and deep glacial sediments of this region result in large available soil storage capacity relative to moisture surpluses or deficits, leading to threshold‐dependent rainfall‐runoff relationships. Rainfall simulation experiments were conducted using large magnitude and high intensity applications to examine the thresholds in precipitation and soil moisture that are necessary to generate lateral flow from hillslope runoff plots representative of Luvisolic soils and an aspen canopy. Two adjacent plots (areas of 2·95 and 3·4 m²) of contrasting antecedent moisture conditions were examined; one had tree root uptake excluded for two months to increase soil moisture content, while the second plot allowed tree uptake over the growing season resulting in drier soils. Vertical flow as drainage and soil moisture storage dominated the water balances of both plots. Greater lateral flow occurred from the plot with higher antecedent moisture content. Results indicate that a minimum of 15–20 mm of rainfall is required to generate lateral flow, and only after the soils have been wetted to a depth of 0·75 m (C‐horizon). The depth and intensity of rainfall events that generated runoff > 1 mm have return periods of 25 years or greater and, when combined with the need for wet antecendent conditions, indicate that lateral flow generation on these hillslopes will occur infrequently. Copyright © 2008 John Wiley & Sons, Ltd.     

Estimating growing-season root zone soil moisture from vegetation index-based evapotranspiration fraction and soil properties in the Northwest Mountain region,USA

A soil moisture retrieval method is proposed, in the absence of ground-based auxiliary measurements, by deriving the soil moisture content relationship from the satellite vegetation index-based evapotranspiration fraction and soil moisture physical properties of a soil type. A temperature–vegetation dryness index threshold value is also proposed to identify water bodies and underlying saturated areas. Verification of the retrieved growing season soil moisture was performed by comparative analysis of soil moisture obtained by observed conventional in situ point measurements at the 239-km² Reynolds Creek Experimental Watershed, Idaho, USA (2006–2009), and at the US Climate Reference Network (USCRN) soil moisture measurement sites in Sundance, Wyoming (2012–2015), and Lewistown, Montana (2014–2015). The proposed method best represented the effective root zone soil moisture condition, at a depth between 50 and 100 cm, with an overall average R² value of 0.72 and average root mean square error (RMSE) of 0.042.     

Characteristics of soil moisture in permafrost observed in East Siberian taiga with stable isotopes of water

Soil moisture and its isotopic composition were observed at Spasskaya Pad experimental forest near Yakutsk, Russia, during summer in 1998, 1999, and 2000. The amount of soil water (plus ice) was estimated from volumetric soil water content obtained with time domain reflectometry. Soil moisture and its δ¹⁸O showed large interannual variation depending on the amount of summer rainfall. The soil water δ¹⁸O decreased with soil moisture during a dry summer (1998), indicating that ice meltwater from a deeper soil layer was transported upward. On the other hand, during a wet summer (1999), the δ¹⁸O of soil water increased due to percolation of summer rain with high δ¹⁸O values. Infiltration after spring snowmelt can be traced down to 15 cm by the increase in the amount of soil water and decrease in the δ¹⁸O because of the low δ¹⁸O of deposited snow. About half of the snow water equivalent (about 50 mm) recharged the surface soil. The pulse of the snow meltwater was, however, less important than the amount of summer rainfall for intra‐annual variation of soil moisture. Excess water at the time just before soil freezing, which is controlled by the amount of summer rainfall, was stored as ice during winter. This water storage stabilizes the rate of evapotranspiration. Soil water stored in the upper part of the active layer (surface to about 120 cm) can be a water source for transpiration in the following summer. On the other hand, once water was stored in the lower part of the active layer (deeper than about 120 cm), it would not be used by plants in the following summer, because the lower part of the active layer thaws in late summer after the plant growing season is over. Copyright © 2002 John Wiley & Sons, Ltd.     

Dynamics of evapotranspiration from a riparian pond complex in the Western Boreal Forest,Alberta, Canada

The relative contributions to total actual evapotranspiration (AET) from pond and riparian areas in a pond‐wetland complex in the Western Boreal Plain (WBP) of northern Alberta are measured using the Bowen ratio energy balance technique. Measurements show that a pond typical of the WBP evaporates at a rate more than twice that of the adjacent riparian peatland. Relating the actual to potential evapotranspiration over both surfaces yields Priestley–Taylor α coefficients of 0·69 and 1·11 for the peatland and pond respectively. Further results demonstrate that the sheltering and turbulent influences of the adjacent forested areas must be considered in the processes governing the permanence of WBP ponds. That is, forestry practices may inadvertently enhance the evaporative losses from the ponds, over and above the controls exerted by the regional climate. Copyright © 2006 John Wiley & Sons, Ltd.     

Development of a continuous soil moisture accounting procedure for curve number methodology and its behaviour with different evapotranspiration methods

The curve number (CN) method is widely used for rainfall–runoff modelling in continuous hydrologic simulation models. A sound continuous soil moisture accounting procedure is necessary for models using the CN method. For shallow soils and soils with low storage, the existing methods have limitations in their ability to reproduce the observed runoff. Therefore, a simple one‐parameter model based on the Soil Conservation Society CN procedure is developed for use in continuous hydrologic simulation. The sensitivity of the model parameter to runoff predictions was also analysed. In addition, the behaviour of the procedure developed and the existing continuous soil moisture accounting procedure used in hydrologic models, in combination with Penman–Monteith and Hargreaves evapotranspiration (ET) methods was also analysed. The new CN methodology, its behaviour and the sensitivity of the depletion coefficient (model parameter) were tested in four United States Geological Survey defined eight‐digit watersheds in different water resources regions of the USA using the SWAT model. In addition to easy parameterization for calibration, the one‐parameter model developed performed adequately in predicting runoff. When tested for shallow soils, the parameter is found to be very sensitive to surface runoff and subsurface flow and less sensitive to ET. Copyright © 2007 John Wiley & Sons, Ltd.     

Factors affecting soil moisture spatial variability for a humid forest hillslope

Soil moisture is an important variable in explaining hydrological processes at hillslope scale. The distribution of soil moisture along a hillslope is related to the spatial distribution of the soil properties, the topography, the soil depth, and the vegetation. In order to investigate the factors affecting soil moisture, various environmental data were collected from a humid forest hillslope in this study. Several factors (the wetness index; the contributing area; the local slope; the soil depth; the composition of sand, silt, and clay; the scaling parameter; the hydraulic conductivity; the tree diameter at breast height; and the total weighted basal area) were evaluated for their effect on soil moisture and its distribution over the hillslope at depths of 10, 30, and 60 cm. Both linear correlation analysis and empirical orthogonal function analysis indicated that the soil texture was a dominant factor in soil moisture distribution. The impact of soil hydraulic conductivity was important for all soil moisture ranges at a depth of 30 cm, but those at 10 and 60 cm were limited to very wet and dry conditions, respectively. The relationships of the various factors with the spatial variability of soil moisture indicated the existence of a threshold soil moisture that is related to the composition of the soil and the factors related to the distribution of water in the study area.     

Groundwater recharge estimation using improved soil moisture balance methodology for a tropical climate with distinct dry seasons

Abstract

Groundwater recharge is estimated using an improved daily soil moisture balance based on a single soil water store for a climate classified as tropical with distinct dry seasons; an upland area in northwest Sri Lanka is used as an example. When the water availability is limited and the soil is under stress, the actual evapotranspiration is less than the potential value; the stress factor is estimated in terms of the readily and total available water, soil properties and effective root depth. Runoff is estimated using coefficients which depend on rainfall intensity and soil moisture deficits. A new component, near surface storage, is used to represent continuing evapotranspiration on days following heavy rainfall even though the soil moisture deficit is high. Recharge is estimated for permanent grass and a commonly cultivated vegetable crop. The plausibility of the model outputs is examined using independent information and data, including well water level fluctuations. Uncertainties and variations in parameter values are explored using sensitivity analyses.     

Seasonal ground ice impacts on spring ecohydrological conditions in a western boreal plains peatland

Peatlands in the Western Boreal Plains act as important water sources in the landscape. Their persistence, despite potential evapotranspiration (PET) often exceeding annual precipitation, is attributed to various water storage mechanisms. One storage element that has been understudied is seasonal ground ice (SGI). This study characterized spring SGI conditions and explored its impacts on available energy, actual evapotranspiration, water table, and near surface soil moisture in a western boreal plains peatland. The majority of SGI melt took place over May 2017. Microtopography had limited impact on melt rates due to wet conditions. SGI melt released 139mm in ice water equivalent (IWE) within the top 30cm of the peat, and weak significant relationships with water table and surface moisture suggest that SGI could be important for maintaining vegetation transpiration during dry springs. Melting SGI decreased available energy causing small reductions in PET (<10mm over the melt period) and appeared to reduce actual evapotranspiration variability but not mean rates, likely due to slow melt rates. This suggests that melting SGI supplies water, allowing evapotranspiration to occur at near potential rates, but reduces the overall rate at which evapotranspiration could occur (PET). The role of SGI may help peatlands in headwater catchments act as a conveyor of water to downstream landscapes during the spring while acting as a supply of water for the peatland. Future work should investigate SGI influences on evapotranspiration under differing peatland types, wet and dry spring conditions, and if the spatial variability of SGI melt leads to spatial variability in evapotranspiration.     

Impacts of climate change on soil moisture and evapotranspiration in reconstructed watersheds in northern Alberta,Canada

This study aims at developing a generalized understanding of the sensitivity of soil moisture patterns in reconstructed watersheds, in northern Alberta, to changes in the projected precipitation in the twenty‐first century. The GSDW model is applied to three watersheds using climate scenarios generated using daily precipitation and air temperature output from a global climate model (CGCM3), under A2 and B1 emission scenarios, to simulate the corresponding soil moisture. CGCM3 results indicate an increase in the mean annual temperature for Fort McMurray, Alberta of 3·3 (A2) and 2·4 °C (B1), and an increase in the predicted annual precipitation of 34% (A2) and 8·6% with A2 and B1 emission scenarios, respectively. The GSDW model is used, along with onsite historical data, to downscale A2 and B1 emission scenarios and to evaluate the future hydrological performance of the designated watersheds with respect to soil moisture deficit and actual evapotranspiration using a probabilistic framework. The forecasted maximum soil moisture deficit values based on A2 and B1 emission scenarios are expected to decrease compared to those based on the current, largely because of the expected increase in precipitation rates, associated with an expected increase in evapotranspiration. Copyright © 2011 John Wiley & Sons, Ltd.     

Hydraulic redistribution and hydrological controls on aspen transpiration and establishment in peatlands following wildfire

In the sub‐humid Western Boreal Plains of Alberta, where evapotranspiration often exceeds precipitation, trembling aspen (Populus tremuloides Michx.) uplands often depend on adjacent peatlands for water supply through hydraulic redistribution. Wildfire is common in the Boreal Plains, so the resilience of the transfer of water from peatlands to uplands through roots immediately following wildfire may have implications for aspen succession. The objective of this research was to characterize post‐fire peatland‐upland hydraulic connectivity and assess controls on aspen transpiration (as a measure of stress and productivity) among landscape topographic positions. In May 2011, a wildfire affected 90,000 ha of north central Alberta, including the Utikuma Region Study Area (URSA). Portions of an URSA glacio‐fluval outwash lake catchment were burned, which included forests and a small peatland. Within 1 year after the fire, aspen were found to be growing in both the interior and margins of this peatland. Across recovering land units, transpiration varied along a topographic gradient of upland midslope (0.42 mm hr^?1) > upland hilltop (0.29 mm hr^?1) > margin (0.23 mm hr^?1) > peatland (0.10 mm hr^?1); similar trends were observed with leaf area and stem heights. Although volumetric water content was below field capacity, P. tremuloides were sustained through roots present, likely before fire, in peatland margins through hydraulic redistribution. Evidence for this was observed through the analysis of oxygen (δ¹⁸O) and hydrogen (δ²H) isotopes where upland xylem and peat core signatures were ?10.0‰ and ?117.8‰ and ?9.2‰ and ?114.0‰, respectively. This research highlights the potential importance of hydraulic redistribution to forest sustainability and recovery, in which the continued delivery of water may result in the encroachment of aspen into peatlands. As such, we suggest that through altering ecosystem services, peatland margins following fire may be at risk to aspen colonization during succession.     

A method for estimating soil moisture storage in regions under water stress and storage depletion: a case study of Hai River Basin,North China

Soil moisture is a consideration for soil conservation, agricultural production and climate modelling. This article presents a simple method for estimating soil moisture storage under water stress and storage depletion conditions. The method is driven by the common agro‐hydrologic variables of precipitation (PPT), irrigation (IRR) and evapotranspiration (ET). The proposed method is successfully tested for the 152 000 km² floodplain region of Hai River Basin using 48 consecutive months (2003–2006) of data. Soil moisture data from global land data assimilation system/Noah land surface model are validated with ground‐truth data from 102 soil moisture monitoring sites. The validated soil moisture is used in combination with in situ groundwater data to quantify total water storage change (TWSC) in the region. The estimated storage change is in turn compared with gravity recovery and climate experiment‐derived TWSC for the study area. The soil moisture and TWSC terms show favourable agreements, with discrepancies of < 10% on the average. While there is no consistent seasonal trend in soil moisture, TWSC shows a strong seasonality. It is low in spring and high in summer. This trend corresponds with the IRR–PPT season in the study area. Change in groundwater and total water storage indicates storage depletion in the basin. Storage depletion in the region is driven mainly by groundwater IRR and ET loss. Despite the low PPT and high ET, there is narrowing seasonal trend in soil moisture. This is achieved at the expense of groundwater storage. IRR pumping has induced extensive groundwater depletion in the basin. It is therefore vital to develop cultivation strategies that aim at limiting IRR pumping and ET loss. Water management practices that not only reduce waste but also ensure high productivity and ecological sustainability could also mitigate storage depletion in the region. These measures could reduce further not only the seasonal trend in soil moisture but also that in groundwater storage. Copyright © 2011 John Wiley & Sons, Ltd.     

Spatial and temporal dynamics of soil moisture after rainfall events along a slope in Regosols of southwest China

High frequency seasonal drought in purple soils (Regosols in FAO taxonomy) of the hilly upland areas of Sichuan basin, China, is one of the key restrictive factors for crop production. In order to manage irrigation and fertilizer application in these soils effectively, the soil water content in a sloped plot with 60 cm soil depth was measured by neutron probe devices to investigate the soil moisture regime during the 1998 rainy season after various amounts of rainfall events. The results showed that variation of soil moisture along the slope positions was highest in the top soil layer during the period of sporadic rainfall that did not induce any runoff. The coefficients of variation of soil moisture at various slope positions (upper, middle, and lower) are 17·36%, 8·95%, 10·25%, 8·58%, 8·05% and 9·21% at the 10 cm, 20 cm, 30 cm, 40 cm, 50 cm and 60 cm soil depths respectively. When surface runoff occurred, the soil moisture dynamics at various positions on the plot were then very different. Soil water content decreased more rapidly on the upper slope than on the middle and lower slope positions. When both surface runoff and throughflow occurred, the soil moisture dynamics in the various layers showed a stable period (soil water content is near constant as time elapses) that lasted about 1 week. Also, the pattern of moisture dynamics is ‘decreasing–stabilization–decreasing’. Thus, irrigation and fertilization management according to the spatial and temporal features of soil moisture dynamics on sloped land can increase the water and fertilizer utilization efficacy by reducing their losses during the stable period. Copyright © 2006 John Wiley & Sons, Ltd.     

Vegetation structure controls on snow and soil moisture in restored ponderosa pine forests

Thinning of semi-arid forests to reduce wildfire risk is believed to improve forest health by increasing soil moisture. Increased snowpack, reduced transpiration and reduced rainfall interception are frequently cited mechanisms by which reduced canopy density may increase soil moisture. However, the relative importance of these factors has not been rigorously evaluated in field studies. We measured snow depth, snow water equivalent (SWE) and the spatial and temporal variation in soil moisture at four experimental paired treatment-control thinning sites in high elevation ponderosa pine forest northern Arizona, USA. We compared snow and soil moisture measurements with forest structure metrics derived from aerial imagery and 3-dimensional lidar data to determine the relationship between vegetation structure, snow and soil moisture throughout the annual hydrologic cycle. Soil moisture was consistently and significantly higher in thinned forest plots, even though the treatments were performed 8–11 years before this study. However, we did not find evidence that SWE was higher in thinned forests across a range of snow conditions. Regression tree analysis of soil moisture and vegetation structure data provided some evidence that localized differences in transpiration and interception of precipitation influence the spatial pattern of soil moisture at points in the annual hydrologic cycle when the system is becoming increasingly water limited. However, vegetation structure explained a relatively low amount of the spatial variance (R² < 0.23) in soil moisture. Continuous measurements of soil moisture in depth profiles showed stronger attenuation of soil moisture peaks in thinned sites, suggesting differences in infiltration dynamics may explain the difference in soil moisture between treatments as opposed to overlying vegetation alone. Our results show limited support for commonly cited relationships between vegetation structure, snow and soil moisture and indicate that future research is needed to understand how reduction in tree density alters soil hydraulic properties.     

Deep soil water dynamics in an undisturbed primary forest in central Amazonia: Differences between normal years and the 2005 drought

Understanding how Amazonian rainforests deal with extended droughts is critical in the face of changing climate. This research analyze the physical properties and the soil water dynamics of a deep soil profile in an area of primary forest in central Amazonia to elucidate these processes under drought and nondrought conditions. Physical soil properties derived from soil cores exhibited a distinctive layer between 480 and 880 cm deep, characterized by higher microporosity and low plant water availability. In situ soil moisture measurements collected during the period from January 2003 through February 2006 and for depths ranging from 10 to 1,430 cm suggest that, in the study site, the top 480 cm of the soil profile satisfied most of the transpirational demands in normal climatological years. However, during exceptionally dry periods, such as the 2005 drought, root uptake occurs below 480 cm. As concluded by previous studies, most of the uptake is concentrated in the first meter of the soil profile: More than 40% of the total demand for transpiration is supplied by the top meter of soil. Because deep root uptake occurred at greater depths than normal during the 2005 drought, our results suggest that this is a fundamental mechanism to cope with prolonged droughts.     

Effects of rainfall on soil moisture and water movement in a subalpine dark coniferous forest in southwestern China

Water content and movement in soil profile and hydrogen isotope composition (δD) of soil water, rainwater, and groundwater were examined in a subalpine dark coniferous forest in the Wolong National Nature Reserve in Sichuan, China, following rainfall events in 2003–2004. Light rainfall increased water content in the litter and at soil depth of 0–80 cm, but the increased soil water was lost in several days. Heavy rainfall increased soil water content up to 85% at depths of 0–40 cm. Following the light rainfall in early spring, the δD of water from the litter, humus, illuvial, and material layers decreased first and then gradually reached the pre‐rainfall level. In summer, light rainfall reached the litter humus, and illuvial layer, but did not hit the material layer. Heavy rainfall affected δD of water in all layers. The δD of soil interflow slightly fluctuated with rainfall events. The δD of shallow groundwater did not differ significantly among all rainfall events. Light rainfall altered the shape of δD profile curve of water in the upper layer of soil, whereas heavy rainfall greatly affected the shape of δD profile curve of water in all soil layers. Following the heavy rainfall, preferential flow initially occurred through macropores, decayed plant roots, and rocks at different depths of soil profile. With continuing rainfall, the litter and surface soil were nearly saturated or fully saturated, and infiltration became homogeneous and plug‐like. Forest soil water, particularly in deeper soil profile, was slightly affected by rainfall and, thus, can be a source of water supply for regional needs, particularly during dry seasons. Copyright © 2011 John Wiley & Sons, Ltd.     

Analysis of the contributions of topographic,soil, and vegetation features on the spatial distributions of surface soil moisture in a steep natural forested headwater catchment

Surface soil moisture has been extensively studied for various land uses and landforms. Although many studies have reported potential factors that control surface soil moisture over space or time, the findings have not always been consistent, indicating a need for identification of the main factors. This study focused on the static controls of topographic, soil, and vegetation features on surface soil moisture in a steep natural forested headwater catchment consisting of three hillslope units of a gully area, side slope, and valley‐head slope. Using a simple correlation analysis to investigate the effects of the static factors on surface soil moisture at depths of 0–20 cm at 470 points in 13 surveys, we addressed the characteristics of surface soil moisture and its main controlling factors. The results indicated that the mean of surface soil moisture was in the decreasing order of gully area > valley‐head slope > side slope. The relationship between the mean and standard deviation of surface soil moisture showed a convex‐upward shape in the headwater catchment, a negative curvilinear shape in the gully area, and positive curvilinear shapes at the side and valley‐head slopes. At the headwater catchment and valley‐head slope, positive contributions of soil porosity and negative contributions of slope gradient and saturated hydraulic conductivity were the main controlling factors of surface soil moisture under wetter conditions, whereas positive contributions of topographic wetness index and negative contributions of vegetation density were the main controlling factors of surface soil moisture under drier conditions. At the side slope underlain by fractured bedrocks, only saturated hydraulic conductivity and vegetation density were observed to be the controlling factors. Surface soil moisture in the gully area was mainly affected by runoff rather than were static features. Thus, using hillslope units is effective for approximately estimating the hydrological behaviours of surface moisture on a larger scale, whereas dependency between the main static factors and moisture conditions is helpful for estimating the spatial distributions of surface moisture on a smaller scale.     

Soil partitioning and surface store controls on spring runoff from a boreal forest peatland basin in north‐central Manitoba,Canada

The mid‐ to high‐boreal forest in Canada occupies the discontinuous permafrost zone, and is often underlain by glaciolacustrine sediments mantled by a highly porous organic mat. The result is a poorly drained landscape dominated by wetlands. Frost‐table dynamics and surface storage conditions help to control runoff contributions from various landscape elements, hydrological linkages between these elements, and basin streamflow during spring snowmelt. Runoff components and pathways in a forested peatland basin were assessed during two spring snowmelts with contrasting input and basin conditions. Runoff from relatively intense melt (up to 16 mm day^?1) on slopes with limited soil thawing combined with large pre‐melt storage in surface depressions to produce high flows composed primarily of meltwater (78% of the 0·29 m³ s^?1 peak discharge) routed over wetland surfaces and through permeable upper peat layers. Melt intensity was less in the subsequent year (maximum of 10 mm day^?1) and active layer development was relatively greater (0·2 m deeper at the end of spring melt), resulting in less slope runoff. Coupling of reduced slope contributions with lower storage levels in basin wetlands led to relatively subdued streamflows dominated by older water (73% of the 0·09 m³ s^?1 peak discharge) routed through less‐permeable deeper peat layers and mineral soil. Interannual differences in runoff conditions provide important insight for the development of distributed hydrological models for boreal forest basins and into potential influences on biogeochemical cycling in this landscape under a warming climate. Copyright © 2001 John Wiley & Sons, Ltd.