The Impact of Variable Horizon Shade on the Growing Season Energy Budget of a Subalpine Headwater Wetland

Richard M Petrone,Myroslava Khomik,Dylan M Hrach,Brandon Van Huizen,Adam Green

doi:10.3390/atmos12111473

Abstract

Surface energy budgets are important to the ecohydrology of complex terrain, where land surfaces cycle in and out of shadows creating distinct microclimates. Shading in such environments can help regulate downstream flow over the course of a growing season, but our knowledge on how shadows impact the energy budget and consequently ecohydrology in montane ecosystems is very limited. We investigated the influence of horizon shade on the surface energy fluxes of a subalpine headwater wetland in the Canadian Rocky Mountains during the growing season. During the study, surface insolation decreased by 60% (32% due to evolving horizon shade and 28% from seasonality). The influence of shade on the energy budget varied between two distinct periods: (1) Stable Shade, when horizon shade was constant and reduced sunlight by 2 h per day; and (2) Dynamic Shade, when shade increased and reduced sunlight by 0.18 h more each day, equivalent to a 13% reduction in incoming shortwave radiation and 16% in net radiation. Latent heat flux, the dominant energy flux at our site, varied temporally because of changes in incoming radiation, atmospheric demand, soil moisture and shade. Horizon shade controlled soil moisture at our site by prolonging snowmelt and reducing evapotranspiration in the late growing season, resulting in increased water storage capacity compared to other mountain wetlands. With the mounting risk of climate-change-driven severe spring flooding and late season droughts downstream of mountain headwaters, shaded subalpine wetlands provide important ecohydrological and mitigation services that are worthy of further study and mapping. This will help us better understand and protect mountain and prairie water resources.

Highlights

Mountain regions have been termed the “water towers of the world” due to their importance in regulating the global water balance and providing a water source disproportionate to their surface area [1,2]
We found that horizon shade had a strong influence at the site by the end of our study period, decreasing sunlight by greater margins than seasonal changes associated with sun angle (Figure 6b)
This study is the first to attempt to quantify the impact of shade on the components of surface energy budget in a shaded, subalpine wetland

Summary

Introduction

Mountain regions have been termed the “water towers of the world” due to their importance in regulating the global water balance and providing a water source disproportionate to their surface area [1,2]. A recent special Intergovernmental Panel on Climate Change report on high mountain areas [7] highlighted that snowpack has been reduced in recent years due to warmer air temperatures experienced at higher elevations These warmer temperatures result in earlier spring snowmelt, which can lead to earlier peak flows. Goulden and Bales [9] found that at elevations above about 2000 m in the Sierra Nevada Mountains, vegetation was limited by cold temperatures instead of soil moisture, like those at lower elevations As a result, they found that warming projected at high elevation sites by year 2100, could increase ET by 28% and decrease river flow by 26% [9]

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