Victor Engel

An approach to understanding hydrologic connectivity on the hillslope and the implications for nutrient transport

[1] Hydrologic processes control much of the export of organic matter and nutrients from the land surface. It is the variability of these hydrologic processes that produces variable patterns of nutrient transport in both space and time. In this paper, we explore how hydrologic ‘‘connectivity’’ potentially affects nutrient transport. Hydrologic connectivity is defined as the condition by which disparate regions on the hillslope are linked via subsurface water flow. We present simulations that suggest that for much of the year, water draining through a catchment is spatially isolated. Only rarely, during storm and snowmelt events when antecedent soil moisture is high, do our simulations suggest that mid-slope saturation (or near saturation) occurs and that a catchment connects from ridge to valley. Observations during snowmelt at a small headwater catchment in Idaho are consistent with these model simulations. During early season discharge episodes, in which the mid-slope soil column is not saturated, the electrical conductivity in the stream remains low, reflecting a restricted, local (lower slope) source of stream water and the continued isolation of upper and mid-slope soil water and nutrients from the stream system. Increased streamflow and higher stream water electrical conductivity, presumably reflecting the release of water from the upper reaches of the catchment, are simultaneously observed when the mid-slope becomes sufficiently wet. This study provides preliminary evidence that the seasonal timing of hydrologic connectivity may affect a range of ecological processes, including downslope nutrient transport, C/N cycling, and biological productivity along the toposequence. A better elucidation of hydrologic connectivity will be necessary for understanding local processes as well as material export from land to water at regional and global scales. INDEX TERMS: 1615 Global Change: Biogeochemical processes (4805); 1860 Hydrology: Runoff and streamflow; 1866 Hydrology: Soil moisture; 1899 Hydrology: General or miscellaneous; KEYWORDS: carbon and nitrogen transport, hydrologic connectivity, TOPMODEL

Forest canopy hydraulic properties and catchment water balance: observations and modeling

We present observations and simulations examining plant–water relations in a forested catchment characterized by strong topographic control over surface hydrology and stand structure. The system is dominated by xeric and mesic ecotypes of Quercus rubra L. Soil depths are typically very thin and the severity of seasonal water stress in these trees is determined largely by position along the local topographic gradient. Water balance related ecotypic differences in Q. rubra were measured during the 2000 growing season at an upland and lowland site. Significant site differences in stomatal conductance, sap flux density, and leaf to sapwood area ratios were observed. However, mid-day leaf water potentials and the leaf area-specific canopy hydraulic conductance were not significantly different despite a 20% increase in soil moisture content, an average 10 m increase in tree height, and a higher leaf area index at the lowland site. These results suggest a close coordination of tree morphology, stand structure, and the hydraulic conductance of the combined soil–root–leaf pathway that governs leaf-level water vapor exchange rates similarly across the topographic gradient. To place these stand-level observations in the context of catchment-scale water balance we linked the SPA canopy model with a 1-D soil column model and TOPMODEL hydrologic formulations. The SPA model was used to represent the canopy because of its specific inclusion of hydraulic constraints on transpiration and leaf water potential. The combined model is spatially explicit in the distribution of ecotype morphology, and calculates transpiration rates for TOPMODEL-derived saturated and unsaturated area fractions within the watershed separately. Model predictions of stomatal conductance, upland latent energy flux, stream discharge, and soil moisture content compared favorably with observations. Sensitivity analyses of canopy model parameters indicate stream discharge in this system is most sensitive to changes in the maximum leaf area index, the minimum leaf water potential, and belowground resistance. Discharge was least sensitive to changes in stem hydraulic conductivity and capacitance. Model results and observations are discussed with respect to adaptations to water stress, hydraulic controls on canopy water use, and ecosystem water use.

Simulation of community metabolism and atmospheric carbon dioxide and oxygen concentrations in Biosphere 2

Abstract The complexity and scale of Biosphere 2 under materially closed conditions represented a unique opportunity to investigate couplings between elemental cycles and community metabolism. For this paper, simulation models were developed to explore individual biome effects on atmospheric composition and carbon cycling inside the enclosure. Results suggest soil respiration rates, light intensity, and acid–base equilibrium control atmospheric carbon dioxide and oxygen under materially closed conditions. Experiments with the overall Combined Biome model indicate: (1) the agriculture biome has a greater effect on atmospheric composition than the other biomes due to its large area, high net productivity, and biomass harvests; (2) the rainforest, which occupies ≈20% of Biosphere 2 area, may be responsible for 50% of total community production and respiration of oxygen; (3) the savannah and wetland biomes may be sources of carbon dioxide in the long term; (4) the ocean biome has less effect on atmospheric composition than the terrestrial biomes; and (5) the desert biome may lower carbon dioxide in the atmosphere as much as 2000 ppm during low light. Diurnal curve analyses of oxygen and carbon dioxide from early 1995 produced an average community gross production rate of 23 g O2/m2 per day, a community respiration rate of 25 g O2/m2 per day, and an average carbon dioxide absorption rate of 0.2 g CO2/m2 per h.

Influence of water management and natural variability on dissolved inorganic carbon dynamics in a mangrove-dominated estuary

High-resolution time series measurements of temperature, salinity, pH and pCO2 were made during the period October 2014-September 2015 at the midpoint of Shark River, a 15km tidal river that originates in the freshwater Everglades of south Florida (USA) and discharges into the Gulf of Mexico. Dissolved inorganic carbon dynamics in this system vary over time, and during this study could be classified into three distinct regimes corresponding to October 2014-February 2015 (a wet to dry season transition period), March-May 2015 (dry period) and July-September 2015 (wet period). Average net longitudinal dissolved inorganic carbon (DIC) fluxes and air-water CO2 fluxes from the Shark River estuary were determined for the three periods. Net DIC fluxes to the coast were estimated to vary between 23.2 and 25.4×105mold-1 with an average daily DIC flux of 24.3×105mold-1 during the year of study. CO2 emissions ranged between 5.5 and 7.8×105mold-1 with an average daily value of 6.4×105mold-1 during the year. The differences in estuarine carbon fluxes during the study period are attributed to differences in the relative importance of hydro-climatological drivers. Results suggest that, during months characterized by reduced rainfall, carbon fluxes are affected by water management via control structures in the upstream Everglades marshes. During months with high rainfall, when culverts are closed and rainfall events are more frequent, carbon fluxes depend more on other forcings, such as rainfall and groundwater discharge.