Advancing Methods to Quantify Actual Evapotranspiration in Stony Soil Ecosystems

Water is undeniably among the most important natural resources and the most critical in semi-arid regions like the Intermountain West of the United States. Such regions are characterized by low precipitation, the majority of which is transferred to the atmosphere from the soil and vegetation as evapotranspiration (ET). Quantification of ET is thus crucial for understanding the balance of water within the region, which is important for efficiently planning the available water resources. This study was motivated towards advancing the estimation of actual ET (ETA) in mountain ecosystems, where the variation in different types of vegetation and non-uniformity of soil including considerable stone content creates challenges for estimating water use as ET. With the aim of addressing the effect of stone content in controlling soil moisture and ET, this study examined the influence of stone content on bulk soil hydraulic properties. An averaging model referred to as a binary mixing model was used to describe the way in which water is held and released in stony soil. This approach was based on the individual hydraulic behavior of the background soil and of the stones within the soil. The effect of soil stone content on ETA was evaluated by accounting for the water retention properties of stones in the soil using a numerical simulation model (HYDRUS-1D). The results revealed overestimation of simulated ETA when effects of stone content were not accounted for in comparison to ETA vi measured by the state-of-the-art “eddy covariance” measurement method for ETA. An even larger-scale model was evaluated, named the Noah-Multiphysics (Noah-MP) land surface model. The land surface model was run using different arrangements of complexity to determine the importance of stone content information on simulation results. The version of the model with information about stone content along with detailed soil properties was able to provide the best Noah-MP prediction of ET. The study suggests that improvement in representation of soil properties including stone content information, can substantially advance the ability of numerical and land surface models to more accurately simulate soil water flow and ETA

Estimating Actual Evapotranspiration from Stony-Soils in Montane Ecosystems

Quantification of evapotranspiration (ET) is crucial for understanding the water balance and for efficient water resources planning. Agricultural settings have received most attention regarding ET measurements while less knowledge is available for actual ET (ETA) in natural ecosystems, many of which have soils containing significant amounts of stones. This study is focused on modelling ETA from stony soil, particularly in montane ecosystems where we estimate the contribution of stone content on water retention properties in soil. We employed a numerical model (HYDRUS-1D) to simulate ETA in natural settings in northern Utah and southern Idaho during the 2015 and 2016 growing seasons based on meteorological and soil moisture measurements at a range of depths. We simulated ETA under three different scenarios, considering soil with (i) no stones, (ii) highly porous stones, and (iii) negligibly porous stones. The simulation results showed significant overestimation of ETA when neglecting stones in comparison to ETA measured by eddy covariance. ETA estimates with negligibly porous stones were lower for all cases due to the decrease in soil water storage compared with estimates made considering highly porous stones. Assumptions of highly porous or negligibly porous stones led to reductions in simulated ETA of between 10% and 30%, respectively, compared with no stones. These results reveal the important role played by soil stones, which can impact the water balance by altering available soil moisture and thus ETA in montane ecosystems

Spatial Analysis of Actual Evapotranspiration Estimates from the iUTAH Climate Station Network

Water demand is increasing whereas the supply is diminishing in many parts of the world. Estimating the loss of water through evapotranspiration (ET) processes in arid montane regions of the Intermountain West is important for efficient planning and management of water resources. In this study reference ET (ETr) was estimated using three different equations (i.e. FAO56-PM, ASCE Standardized -PM, Hargreaves-Samani; HS), which were compared with ET measured using the eddy covariance (EC) technique at the T.W. Daniel Experimental Forest (TWDEF) for the years 2010 and 2011. The HS method overestimated ETr by almost two times, while the FAO56-PM equation provided the closest estimates of ETr. The estimated ET values were compared as a function of elevation and found to be decreasing with increased elevation. The point estimates of ETr using the FAO56-PM equation were interpolated using the inverse distance weighting (IDW) method to provide temporal and spatial analysis of actual ET (ETa within iUTAH\u27s (innovative Urban Transitions and Aridregion Hydro-sustainability) monitored watersheds. The ETa values were estimated as the proportion of ETr contributed by the fraction of vegetation derived from the Normalized Difference Vegetation Index (NDVI) computed at each pixel of Landsat images presented as a map for each watershed in May, June, July and August of 2014

A Binary Mixing Model for Characterizing Stony-Soil Water Retention

A century of research focused primarily on agricultural soils has largely ignored stony soils, which dominate some forests and are poorly understood in terms of the stone influence on soil hydraulic properties. Motivated by this knowledge gap, we quantified the influence of soil-containing stone fragments on bulk soil hydraulic properties by determining the water retention curve (WRC) of soil, stone and stone-soil mixtures with varied volumetric stone content. The measured WRC for seven different stone types based on their composition showed maximum and minimum saturated water contents of 0.55 m3 m−3 in pumice and 0.025 m3 m−3 in fine sandstone, respectively. The stony soil water retention function was measured using the simplified evaporation method. Contrasting scenarios were studied considering a broad range of stone inclusions; (i) negligibly porous, (ii) significantly porous but less porous than the background soil, (iii) more porous than the background soil. An averaging scheme to describe the WRC of stony soil was proposed based on the individual WRC of the background and stone inclusion which was in good agreement with the experimental data. The HYDRUS-3D model was also employed to simulate the evaporation experiment used for the WRC measurements. The model simulations supported the basic assumptions of the proposed averaging scheme

Influence of Stone Content in Soil Water Retention

A century of research focused primarily on agricultural soils has largely avoided stony soils, which are poorly understood in terms of their water retention- and hydraulic conductivity-functions. These key functions influence the majority of soil properties and processes and therefore measuring and modeling the influence of rock fragments becomes important for understanding stony soil systems. Relatively few studies have addressed this important problem using physically-based concepts. Motivated by this knowledge gap, we set out to describe soil hydraulic properties using binary mixtures (i.e. rock fragment inclusions in a soil matrix) based on individual properties of the rock and soil. As a first step of this study, special attention was given to the water retention curve (WRC), where the impact of rock content on water-storage and -energy was quantified using laboratory experiments for five different mixing ratios of a silt loam soil matrix and different rock. The WRC for each mixture was obtained from sample mass and water potential measurements. The resulting family of WRCs for the examined mixtures demonstrated how the WRC evolves from the discrete soil and rock WRCs

Insights on the Impacts of Hydroclimatic Extremes and Anthropogenic Activities on Sediment Yield of a River Basin

Streamflow and sediment flux variations in a mountain river basin directly affect the downstream biodiversity and ecological processes. Precipitation is expected to be one of the main drivers of these variations in the Himalayas. However, such relations have not been explored for the mountain river basin, Nepal. This paper explores the variation in streamflow and sediment flux from 2006 to 2019 in central Nepal’s Kali Gandaki River basin and correlates them to precipitation indices computed from 77 stations across the basin. Nine precipitation indices and four other ratio-based indices are used for comparison. Percentage contributions of maximum 1-day, consecutive 3-day, 5-day and 7-day precipitation to the annual precipitation provide information on the severity of precipitation extremeness. We found that maximum suspended sediment concentration had a significant positive correlation with the maximum consecutive 3-day precipitation. In contrast, average suspended sediment concentration had significant positive correlations with all ratio-based precipitation indices. The existing sediment erosion trend, driven by the amount, intensity, and frequency of extreme precipitation, demands urgency in sediment source management on the Nepal Himalaya’s mountain slopes. The increment in extreme sediment transports partially resulted from anthropogenic interventions, especially landslides triggered by poorly-constructed roads, and the changing nature of extreme precipitation driven by climate variability