Erkan Istanbulluoglu

Modeling of the interactions between forest vegetation, disturbances, and sediment yields

in the Idaho batholith are investigated through numerical modeling. The model simulates soil development based on continuous bedrock weathering and the divergence of diffusive sediment transport on hillslopes. Soil removal is due to episodic gully erosion, shallow landsliding, and debris flow generation. In the model, forest vegetation provides root cohesion and surface resistance to channel initiation. Forest fires and harvests reduce the vegetation. Vegetation loss leaves the land susceptible to erosion and landsliding until the vegetation cover reestablishes in time. Simulation results compare well with field observations of event sediment yields and long-term averages over � 10,000 years. When vegetation is not disturbed by wildfires over thousands of years, sediment delivery is modeled to be less frequent but with larger event magnitudes. Increased values of root cohesion (representing denser forests) lead to higher event magnitudes. Wildfires appear to control the timing of sediment delivery. Compared to undisturbed forests, erosion is concentrated during the periods with low erosion thresholds, often called accelerated erosion periods, following wildfires. Our modeling suggests that drainage density is inversely proportional to root cohesion and that reduced forest cover due to wildfires increases the drainage density. We compare the sediment yields under anthropogenic (harvest) and natural (wildfire) disturbances. Disturbances due to forest harvesting appear to increase the frequency of sediment delivery; however, the sediment delivery following wildfires seems to be more severe. These modeling-based findings have implications for engineering design and environmental management, where sediment inputs to streams and the fluctuations and episodicity of these inputs are of concern. INDEX TERMS: 1625 Global Change: Geomorphology and weathering (1824, 1886); 1815 Hydrology: Erosion and sedimentation; 1824 Hydrology: Geomorphology (1625); KEYWORDS: sediment yield, wildfires, forest management, hydrology

Headwater channel dynamics in semiarid rangelands, Colorado high plains, USA

Incised ephemeral channels provide a window into the fluvial processes that help sculpt rangeland landscapes. This paper presents observations of ephemeral channels and valley networks in the high plains of Colorado, USA, with an eye toward painting a picture of the ingredients that must be included in mathematical models of landscape evolution in such environments. Channel incision in the study area is driven by summer thunderstorms, which can with reasonable frequency (3–5 yr) generate boundary shear stresses high enough to penetrate the highly resistant vegetation armor, but only within erosional hot spots where hydraulic forces are amplified by channel constriction and locally steep gradients. Focusing of erosion at these hot spots (which correspond to knickpoints and channel heads) is amplified by the small areal footprint and short “erosional reach” of most convective storms. Upstream migration of knickpoints creates a pattern of short, active channel reaches separated by unchanneled or weakly channeled, fully vegetated stable reaches. Based on our observations, we interpret the necessary and sufficient conditions leading to the observed channel forms and dynamics as: (1) a resistant vegetation layer overlying an erodible substrate, which sets up a conditional instability through which erosional perturbations can grow by positive feedback; (2) high flow variability; (3) moderate to high substrate cohesion; and (4) a high volume fraction of fine-grained erodible material. Concave-upward valley long profiles are interpreted as a trade-off between downstream-increasing flood frequency and downstream-decreasing flood effectiveness. The observed process dynamics imply that long-term rates of valley incision should be especially sensitive to climatic oscillations between episodes of drought and warm-season convective rainfall.

Improving the theoretical underpinnings of process‐based hydrologic models

In this Commentary, we argue that it is possible to improve the physical realism of hydrologic models by making better use of existing hydrologic theory. We address the following questions: (1) what are some key elements of current hydrologic theory; (2) how can those elements best be incorporated where they may be missing in current models; and (3) how can we evaluate competing hydrologic theories across scales and locations? We propose that hydrologic science would benefit from a model-based community synthesis effort to reframe, integrate, and evaluate different explanations of hydrologic behavior, and provide a controlled avenue to find where understanding falls short.

Ecohydrologic role of solar radiation on landscape evolution

Solar radiation has a clear signature on the spatial organization of ecohydrologic fluxes, vegetation patterns and dynamics, and landscape morphology in semiarid ecosystems. Existing landscape evolution models (LEMs) do not explicitly consider spatially explicit solar radiation as model forcing. Here, we improve an existing LEM to represent coupled processes of energy, water, and sediment balance for semiarid fluvial catchments. To ground model predictions, a study site is selected in central New Mexico where hillslope aspect has a marked influence on vegetation patterns and landscape morphology. Model predictions are corroborated using limited field observations in central NM and other locations with similar conditions. We design a set of comparative LEM simulations to investigate the role of spatially explicit solar radiation on landscape ecohydro-geomorphic development under different uplift scenarios. Aspect-control and network-control are identified as the two main drivers of soil moisture and vegetation organization on the landscape. Landscape-scale and long-term implications of these short-term ecohdrologic patterns emerged in modeled landscapes. As north facing slopes (NFS) get steeper by continuing uplift they support erosion-resistant denser vegetation cover which leads to further slope steepening until erosion and uplift attains a dynamic equilibrium. Conversely, on south facing slopes (SFS), as slopes grow with uplift, increased solar radiation exposure with slope supports sparser biomass and shallower slopes. At the landscape scale, these differential erosion processes lead to asymmetric development of catchment forms, consistent with regional observations. Understanding of ecohydrogeomorphic evolution will improve to assess the impacts of past and future climates on landscape response and morphology.

Solar radiation as a global driver of hillslope asymmetry: Insights from an ecogeomorphic landscape evolution model

Observations at the field, catchment, and continental scales across a range of arid and semiarid climates and latitudes reveal aspect-controlled patterns in soil properties, vegetation types, ecohydrologic fluxes, and hillslope morphology. Although the global distribution of solar radiation on earths surface and its implications on vegetation dynamics are well documented, we know little about how variation of solar radiation across latitudes influence landscape evolution and resulting geomorphic difference. Here, we used a landscape evolution model that couples the continuity equations for water, sediment, and aboveground vegetation biomass at each model element in order to explore the controls of latitude and mean annual precipitation (MAP) on the development of hillslope asymmetry (HA). In our model, asymmetric hillslopes emerged from the competition between soil creep and vegetation-modulated fluvial transport, driven by spatial distribution of solar radiation. Latitude was a primary driver of HA because of its effects on the global distribution of solar radiation. In the Northern Hemisphere, north-facing slopes (NFS), which support more vegetation cover and have lower transport efficiency, get steeper toward the North Pole while south-facing slopes (SFS) get gentler. In the Southern Hemisphere, the patterns are reversed and SFS get steeper toward the South Pole. For any given latitude, MAP is found to have minor control on HA. Our results underscore the potential influence of solar radiation as a global control on the development of asymmetric hillslopes in fluvial landscapes.

Predicting glacio‐hydrologic change in the headwaters of the Zongo River, Cordillera Real, Bolivia

In many partially glacierized watersheds glacier recession driven by a warming climate could lead to complex patterns of streamflow response over time, often marked with rapid increases followed by sharp declines, depending on initial glacier ice cover and rate of climate change. Capturing such “phases” of hydrologic response is critical in regions where communities rely on glacier meltwater, particularly during low flows. In this paper, we investigate glacio-hydrologic response in the headwaters of the Zongo River, Bolivia, under climate change using a distributed glacio-hydrological model over the period of 1987-2100. Model predictions are evaluated through comparisons with satellite-derived glacier extent estimates, glacier surface velocity, in-situ glacier mass balance, surface energy flux, and stream discharge measurements. Historically (1987-2010) modeled glacier melt accounts for 27% of annual runoff, and 61% of dry season (JJA) runoff on average. During this period the relative glacier cover was observed to decline from 35% to 21% of the watershed. In the future, annual and dry season discharge is projected to decrease by 4% and 27% by midcentury and 25% and 57% by the end of the century, respectively, following the loss of 81% of the ice in the watershed. Modeled runoff patterns evolve through the interplay of positive and negative trends in glacier melt and increased evapotranspiration as the climate warms. Sensitivity analyses demonstrate that the selection of model surface energy balance parameters greatly influences the trajectory of hydrological change projected during the first half of the 21st century. These model results underscore the importance of coupled glacio-hydrology modeling. This article is protected by copyright. All rights reserved.

Reply to comment by Jonathan J. Rhodes on “Modeling of the interactions between forest vegetation, disturbances, and sediment yields”

] Rhodes [2005] brings up some excellent points in hiscomments on the work of Istanbulluoglu et al. [2004]. Weappreciate the opportunity to respond because it is likelythat other readers will also wonder how they can apply therelatively simple analysis to important policy questions.Models necessarily reduce the complexity of the problemto make it tractable and synthesize some diverse sources ofinformation. It may be helpful at times for readers tounderstand the high dimension of the complexity sacrificedin order to obtain the synthesis and the reasons for reducingthe complexity in a particular manner. Rhodes [2005] com-ments on three things: (1) the omission of roads and land-ings from the analysis; (2) the implicit assumption that firedoes not occur with harvesting; and (3) the overestimationof water repellency. We will respond to each of these,clarifying and elaborating on the basis for our modelingchoices.

Which way do you lean? Using slope aspect variations to understand Critical Zone processes and feedbacks: Which way do you lean?

Soil-mantled pole-facing hillslopes on Earth tend to be steeper, wetter, and have more vegetation cover compared with adjacent equator-facing hillslopes. These and other slope aspect controls are often the consequence of feedbacks among hydrologic, ecologic, pedogenic, and geomorphic processes triggered by spatial variations in mean annual insolation. In this paper we review the state of knowledge on slope aspect controls of Critical Zone (CZ) processes using the latitudinal and elevational dependence of topographic asymmetry as a motivating observation. At relatively low latitudes and elevations, pole-facing hillslopes tend to be steeper. At higher latitudes and elevations this pattern reverses. We reproduce this pattern using an empirical model based on parsimonious functions of latitude, an aridity index, mean-annual temperature, and slope gradient. Using this empirical model and the literature as guides, we present a conceptual model for the slope-aspect-driven CZ feedbacks that generate asymmetry in water-limited and temperature-limited end-member cases. In this conceptual model the dominant factor driving slope aspect differences at relatively low latitudes and elevations is the difference in mean-annual soil moisture. The dominant factor at higher latitudes and elevations is temperature limitation on vegetation growth. In water-limited cases, we propose that higher mean-annual soil moisture on polefacing hillslopes drives higher soil production rates, higher water storage potential, more vegetation cover, faster dust deposition, and lower erosional efficiency in a positive feedback. At higher latitudes and elevations, pole-facing hillslopes tend to have less vegetation cover, greater erosional efficiency, and gentler slopes, thus reversing the pattern of asymmetry found at lower latitudes and elevations. Our conceptual model emphasizes the linkages among shortand long-timescale processes and across CZ sub-disciplines; it also points to opportunities to further understand how CZ processes interact. We also demonstrate the importance of paleoclimatic conditions and non-climatic factors in influencing slope aspect variations. Copyright

Nutrient Loss Following Phragmites australis Removal in Controlled Soil Mesocosms

Mechanisms to remove Common reed (Phragmites australis) typically include a combination of herbicide applications and mechanical cutting or plowing of the soil. The objective of this study was to remove P. australis by various mechanisms and measure the subsequent short-term release of nutrients via simulated rain events. Three rain events of similar duration and intensity were conducted on a control subset and three treatments (above and belowground biomass removal, herbicide application, and basal cut) of soil mesocosms (n = 6) that were designed to export excess water as either surface runoff or leachate through the soil profile. The dominant pathway for soluble reactive phosphorus (p < 0.001) and ammonium (p < 0.001) export were surface runoff while nitrate (p < 0.001) was leached through the soil profile. More nitrate was exported in the vegetation removal treatments (i.e., biomass removal, herbicide, and basal cut) than the control (p < 0.001) while more soluble reactive phosphorus was exported in the herbicide and basal cut treatment compared to the control (p = 0.010). In regards to ammonium, a higher export was observed in the herbicide treatment compared to the control, biomass removal, and basal cut treatments (p < 0.001). We attribute the higher amount of ammonium export in the herbicide treatment to the fact that the glyphosate herbicide used was in an isopropylamine salt form. After examining pre- and postmanipulation soil cores, there was a larger decrease in extractable ammonium in the control and all treatments compared to soil extractable nitrate, which displayed a smaller decrease and in some treatments actually increased during the course of the experiment. Ultimately, in this study, we observed a strong potential for nitrogen biogeochemistry to occur and the removal of vegetation-enhanced nutrient export.

Glacier Recession and the Response of Summer Streamflow in the Pacific Northwest United States, 1960–2099

The Pacific Northwest is the most highly glacierized region in the conterminous United States (858 glaciers; 466 km). These glaciers have displayed ubiquitous patterns of retreat since the 1980s mostly in response to warming air temperatures. Glacier melt provides water for downstream uses including agricultural water supply, hydroelectric power generation, and for ecological systems adapted to cold reliable streamflow. While changes in glacier area have been studied within the region over an extended period of time, the hydrologic consequences of these changes are not well defined. We applied a high-resolution glacio-hydrological model to predict glacier mass balance, glacier area, and river discharge for the period 1960–2099. Six river basins across the region were modeled to characterize the regional hydrological response to glacier change. Using these results, we generalized past and future glacier area change and discharge across the entire Pacific Northwest using a k-means cluster analysis. Results show that the rate of regional glacier recession will increase, but the runoff from glacier melt and its relative contribution to streamflow display both positive and negative trends. In high-elevation river basins enhanced glacier melt will buffer strong declines in seasonal snowpack and decreased late summer streamflow, before the glaciers become too small to support streamflow at historic levels later in the 21st century. Conversely, in lower-elevation basins, smaller snowpack and the shrinkage of small glaciers result in continued reductions in summer streamflow.