Representing nighttime and minimum conductance in CLM4.5: global hydrology and carbon sensitivity analysis using observational constraints

Abstract

The terrestrial biosphere regulates climate through carbon, water, and energy exchanges with the atmosphere. Land-surface models estimate plant transpiration, which is actively regulated by stomatal pores, and provide projections essential for understanding Earth's carbon and water resources. Empirical evidence from 204 species suggests that significant amounts of water are lost through leaves at night, though land-surface models typically reduce stomatal conductance to nearly zero at night. Here, we test the sensitivity of carbon and water budgets in a global land-surface model, the Community Land Model (CLM) version 4.5, to three different methods of incorporating observed nighttime stomatal conductance values. We find that our modifications increase transpiration by up to 5% globally, reduce modeled available soil moisture by up to 50% in semi-arid regions, and increase the importance of the land surface in modulating energy fluxes. Carbon gain declines by up to ∼ 4% globally and > 25% in semi-arid regions. We advocate for realistic constraints of minimum stomatal conductance in future climate simulations, and widespread field observations to improve parameterizations.

Geosci. Model Dev., 10, 321331, 2017 www.geosci-model-dev.net/10/321/2017/ doi:10.5194/gmd-10-321-2017 Author(s) 2017. CC Attribution 3.0 License.

Representing nighttime and minimum conductance in CLM4.5: global hydrology and carbon sensitivity analysis using observational constraints

Danica L. Lombardozzi1, Melanie J. B. Zeppel2, Rosie A. Fisher1, and Ahmed Tawk1,3

1National Center for Atmospheric Research, Boulder, CO, USA

2Department of Biological Sciences, Macquarie University, Sydney, Australia

3Center for Ocean-Land-Atmosphere Studies, George Mason University, Fairfax, VA, USA Correspondence to: Danica L. Lombardozzi ([email protected])

Received: 31 October 2015 Published in Geosci. Model Dev. Discuss.: 4 December 2015 Revised: 18 October 2016 Accepted: 19 December 2016 Published: 23 January 2017

Abstract. The terrestrial biosphere regulates climate through carbon, water, and energy exchanges with the atmosphere. Land-surface models estimate plant transpiration, which is actively regulated by stomatal pores, and provide projections essential for understanding Earths carbon and water resources. Empirical evidence from 204 species suggests that signicant amounts of water are lost through leaves at night, though land-surface models typically reduce stomatal conductance to nearly zero at night. Here, we test the sensitivity of carbon and water budgets in a global land-surface model, the Community Land Model (CLM) version 4.5, to three different methods of incorporating observed nighttime stomatal conductance values. We nd that our modications increase transpiration by up to 5 % globally, reduce modeled available soil moisture by up to 50 % in semi-arid regions, and increase the importance of the land surface in modulating energy uxes. Carbon gain declines by up to 4 % glob

ally and > 25 % in semi-arid regions. We advocate for realistic constraints of minimum stomatal conductance in future climate simulations, and widespread eld observations to improve parameterizations.

1 Introduction

Terrestrial plants must balance their need to obtain CO2 with the risk of desiccation if transpiration continues unchecked. Higher plants evolved stomatal pores to control the exchange of water and carbon between the leaf interior and the atmo-

sphere (Hetherington and Woodward, 2003). Stomatal function is thus the dominant control over terrestrial uxes of water and carbon. Most large-scale land-surface models use an empirical representation of stomatal conductance (gs), similar to the BallWoodrowBerry (BWB) model (Ball, 1988;Ball et al., 1987; Collatz et al., 1991; Leuning, 1995; Medlyn et al., 2011; Sellers et al., 1996), to calculate plantgas exchange. The BWB model is linear, with two constants, the intercept (go) and slope (g1), and it estimates gs from the rate of CO2 assimilation (A), atmospheric humidity (hr), and internal leaf CO2 concentration. The original BWB model parameters were tted to observations of leafgas exchange for 10 plant species, with different go values for each species, ranging from 310 to 130 mmol m2 s1 (Ball, 1988). The

Community Land Model (CLM), however, uses only two go values, (10 and 40 mmol m2 s1 for C3 plants and C4 plants, respectively; Collatz et al., 1991; Oleson et al., 2013; Sellers et al., 1996). Conductance during the night (and at other times when A is 0) is thus represented using go. Recent advances in our ability to observe nighttime stomatal conductance (Caird et al., 2007; Phillips et al., 2010), gs,n, illustrate that values are often larger in the eld than the BWB parameters used in the CLM.

A comprehensive database (see Table S1 in the Supplement) of 204 observed gs,n values illustrates that the minimum BWB gs values (equivalent to go) used in the CLM differ starkly from observed mean and median gs,n values.The available data for gs,n range from 0 to 450 mmol m2 s1 with an overall mean of 78 mmol m2 s1 (excluding hemi-parasites and CAM plants, which were omitted from

Published by Copernicus Publications on behalf of the European Geosciences Union.

322 D. L. Lombardozzi et al.: Representing nighttime and minimum conductance in CLM4.5

Table 1. Old and new minimum stomatal conductance values used in CLM4.5SP. Units are mmol m2 s1.

Plant functional type Old Mean new Median new Standard n value value value deviation

temperate needle-leaf evergreen tree 10 16.896 10 20.803 12 boreal needle-leaf evergreen tree 10 8 8 n/a 1 needle-leaf deciduous tree 10 35.367 35 6.458 3 tropical broadleaf evergreen tree 10 90.488 75.5 67.850 8 temperate broadleaf evergreen tree 10 34.017 27 28.263 25 tropical broadleaf deciduous tree 10 129 129 41.012 2 temperate broadleaf deciduous tree 10 72.637 41.66 83.525 22 boreal broadleaf deciduous tree 10 50 50 n/a 1 broadleaf evergreen shrub 10 65.353 29 116.062 16 broadleaf deciduous shrub 10 129.644 60 145.539 9 c3 grass 10 157.988 161 67.317 24 C4 grass 40 93.933 48.5 125.533 6 crop 10 60.629 36.7 60.745 21

150

* New values, standard deviation, and n are based on data pooled from the literature. n/a not applicable

2 Methods

2.1 Model description and simulation design

The CLM4.5SP model used here is an updated version of CLM4.0, originally described by Lawrence et al. (2011), with updated technical details for v4.5 described by Oleson et al. (2013). The CLM4.5SP simulations were run with CRU-NCEP climate forcing data, which combines Climate Research Unit (CRU) TS 3.2 monthly climatology with National Oceanic and Atmospheric Administration National Center for Environmental Prediction (NCEP) and NCAR 2.5 2.5 6-hourly reanalysis (down

loaded at: http://dods.ipsl.jussieu.fr/igcmg/IGCM/BC/OOL/OL/CRU-NCEP/

Web End =http://dods.ipsl.jussieu.fr/igcmg/IGCM/BC/OOL/

http://dods.ipsl.jussieu.fr/igcmg/IGCM/BC/OOL/OL/CRU-NCEP/

Web End =OL/CRU-NCEP/ http://dods.ipsl.jussieu.fr/igcmg/IGCM/BC/OOL/OL/CRU-NCEP/

Web End = ). This is a historical atmospheric dataset that includes observed precipitation, temperature, downward solar radiation, surface wind speed, specic humidity, and air pressure from 1901 through 2010, and did not include the inuences of nitrogen deposition, land-use change, or changing CO2 concentrations.

The CLM4.5SP uses the coupled Farquhar photosynthesis and BWB gs models to simulate plant physiology (Bonan et al., 2011; Oleson et al., 2013). The BWB gs is calculated based on the following equation:

gs = g0 soil + g1(Ahr/Ca), (1)

where g0 and g1 are empirical tting parameters of the minimum gs and the slope of the conductancephotosynthesis relationship, respectively, A is net carbon assimilation rate (mol C m2 s1), hr is the fractional humidity at the leaf surface (dimensionless), Ca is the CO2 concentration at the leaf surface (mol mol1), and soil is the soil wetness scalar, ranging from zero to one (see Oleson et al., 2013). soil is

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model testing). Observations of gs,n are, on average, 10 times higher in broadleaf tropical deciduous species (Table 1; 129 mmol m2 s1) and 7 times higher in temperate broadleaf deciduous trees (73 mmol m2 s1) compared to the 10 mmol m2 s1 used for C3 plants. Potential benets of a high gs,n might include the transport of nutrients (de Dios et al., 2013; Scholz et al., 2007; Zeppel et al., 2014) or processes related to embolism repair, phloem transport, or xylem relling that might improve carbon gain, but these ideas remain untested. Nonetheless, the discrepancy between parameterized go and observed gs,n serves as motivation to investigate the sensitivity of simulated land-surface processes to more realistic minimum gs values. Such eld measurements of gs,n have not previously been incorporated into a global land-surface model, despite the possible impacts on surface hydrology, ecosystem carbon gain, and landatmosphere feedbacks.

We use a global land-surface model, the Community Land Model (CLM) version 4.5, forced with a data atmosphere and driven with observed (satellite phenology) leaf area indices (CLM4.5SP), to test the sensitivity of the land surface to using realistic minimum gs from observed gs,n, averaged by plant functional type (PFT; Table 1). Since the BWB approach is primarily intended to predict daytime stomatal behavior, the appropriate method for application of ob-served gs,n within the context of the BWB model is unclear.

We therefore test three methodologies for implementing ob-served gs,n: (1) modifying the BWB intercept (go), (2) setting a nighttime threshold value, and (3) setting a minimum threshold value. We anticipate that implementing observed gs,n values will increase plant transpiration, altering carbon and water budgets on regional and global scales.

D. L. Lombardozzi et al.: Representing nighttime and minimum conductance in CLM4.5 323

calculated as follows:

soil = [Sigma1]iwiri, (2) where wi is a plant wilting factor for layer i and ri is the fraction of roots in layer i. When implemented in the unmodied CLM4.5SP, g0 is 10 mmol m2 s1 for all C3 plants and 40 mmol m2 s1 for all C4 plants, and is adjusted by soil (varying from 0 to 1) at every time step. It is also important to note that soil is also applied to the Vc,max (the maximum rate of carboxylation) parameter in the A equation, as well as to leaf maintenance respiration (Oleson et al., 2013).

Values of gs,n based on literature data (Table S1 in the Supplement) are typically larger than the g0 values used in current implementations of the BWB model. The gs,n data, grouped and then averaged by PFT (Table 1), were used to modify simulated minimum gs using three methodologies.

First, the [Delta1]g0 method replaced the BWB minimum conductance, g0, value for each PFT with the observed gs,n (Table 1), resulting in a uniform increase to gs during both day and night (referred to as the [Delta1]go simulation; method tested previously by Barnard and Bauerle, 2013). Second, the [Delta1]gnight method implemented the BWB model in its standard form (Eq. 1; the go and g1 values are the same as the control), but included a minimum threshold that was applied only at night, based on observed gs,n for each PFT, below which gs could not fall. In the [Delta1]gnight simulation, daytime [Delta1]gs occasionally fell below the observed nighttime threshold on account of high vapor pressure decit (VPD) or low assimilation rates. To avoid this potentially unrealistic behavior, we use a third method, [Delta1]gmin, which extended the observation-based threshold used in the [Delta1]gnight simulation to all times during the day or night, so that gs never fell below the minimum threshold value found in Table 1. These three modied simulations were compared to a control simulation using the unmodied BWB formulation. Similar to the unmodied and [Delta1]go simulations that adjust the go parameter based on a soil wetness scalar ( soil), the [Delta1]gnight and [Delta1]gmin modications also adjusted the minimum gs threshold by soil at every time step. Each simulation was run for 25 years with monthly output to determine the long-term impact of changing minimum conductance, and for 1 year with half-hourly output to determine the changes in diel patterns.

2.2 Data collection

Values of gs,n were obtained from eld and glasshouse studies, using Scopus (http://www.scopus.com

Web End =www.scopus.com ), with data for 204 records across 150 species and cultivars (Table S1).Records available were predominately for temperate plants (93 records) and crops (34), with more data available for broad-leaf plant types (89) than needle-leaf plants (16; Zeppel et al., 2014). The data were collated by plant functional type (PFT), with means, medians, and standard deviations for each PFT presented in Table 1. Simulations presented here were run with mean values for each PFT, though median val-

ues were also tested and are presented in Figs. S3 and S4 in the Supplement. Since there is large variability in the PFT responses, we present the range of variability in Fig. S2.

The measurements of each gs,n value are generally obtained from steady-state porometers, diffusion porometers, Licor 1600 and Licor 6400 gas exchange systems (Caird et al., 2007; Phillips et al., 2010), with a small number converted from sap ux (Benyon, 1999) using an inverted PenmanMonteith equation. Different sampling methods may lead to different estimates of gs,n, and measurable gs,n typically only occurs where VPD is above zero. For example, using a cuvette clamped over the leaf, which changes the leaf boundary layers, will be different compared to measurements from sap ow with an unaltered boundary layer.Data for gs,n were typically reported during well-watered conditions, which is ideal because the CLM4.5 calculates stomatal gs without water stress and then adjusts go values (and modications additionally adjust gnight and gmin thresholds) using a soil wetness scalar.

2.3 Terrestrial coupling index

To investigate the impact of stomatal conductance changes on the atmosphere, a terrestrial coupling index was calculated, allowing examination of the inuence of a minimum gs threshold on landatmosphere coupling. Following

Dirmeyer (2011), the terrestrial segment of landatmosphere coupling is dened as follows:

Terrestrial coupling index (TCI) = w w,ET, (3) where w is the standard deviation of root-zone soil moisture relevant for transpiration across a given season (e.g., 25 years times 3 summer months), and w,ET is the linear slope of monthly mean evapotranspiration and root-zone soil moisture. The TCI captures the variability (w) and sensitivity of evapotranspiration to changes in soil moisture and returns units equivalent to those of evapotranspiration. Therefore, for a region to have high TCI, soil moisture must have high variability, thus enabling any evapotranspirationsoil moisture sensitivity to manifest in the climate system. While this is strictly a metric for dening the terrestrial component of coupling, the terrestrial component has been used as a surrogate for the total soil moistureprecipitation coupling pattern because of the strong spatial pattern correlation (Wei and Dirmeyer, 2012).

3 Results and discussion

3.1 Implementation of gs,n

Incorporating observed minimum constraints on gs in all modied simulations increased gs and transpiration compared to the control simulation, illustrated in Fig. 1 for a highly impacted semi-arid location in Ethiopia (see Fig. S1

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324 D. L. Lombardozzi et al.: Representing nighttime and minimum conductance in CLM4.5

Figure 1. Diurnal time series of canopy conductance (a, c) and transpiration (b, d) for Ethiopia over 5 days in mid-January (ab) and mid-July (cd). The control simulation (solid black line) had lower conductance and transpiration than the [Delta1]go simulation (dotted red line) and the [Delta1]gmin simulation (dashed blue line). The [Delta1]gnight simulation (dot-dashed teal line) had higher nighttime conductance and transpiration than the control simulation, but similar daytime conductance and transpiration, allowing for daytime conductance to fall below the nighttime threshold. The [Delta1]go simulation added the observed gs,n values to the conductance calculation at every time, day or night, which is not theoretically aligned with the function of including observed gs,n. As a result, the [Delta1]go simulation was eliminated from further analyses.

Note that all minimum thresholds (go, gnight, and gmin) were adjusted using a soil moisture scalar.

for other regions). The large variability in the observational dataset causes substantial uncertainty in the simulations, masking the differences among parameterizations and highlighting the impact of gs,n on transpiration (Fig. S2). The sensitivity of gs and transpiration to the altered go parameter in the [Delta1]go simulation is large (Barnard and Bauerle, 2013; Bowden and Bauerle, 2008). Since the higher go is added to gs in the BWB calculation at every model time step (see

Eq. 1), altering go increases transpiration throughout the entire diel cycle, and produces changes in the daytime evaporative ux that are not supported by observations of gs,n. We

consider that uniformly adjusting the go parameter does not represent the correct implementation of observed gs,n values.

If go cannot be equated to plant minimum gs in the BWB paradigm, this raises the possibility of whether go has a theoretical interpretation beyond an empirical tting parameter. It is possible that go is equivalent to cuticular conductance (gcut), or conductance that is not regulated by the stomatal guard cells (Caird et al., 2007), occurring during the day and night. Niyogi and Raman (1997) describe go as cuticular conductance, though there is no record of go being tested or described as gcut previously. Studies that

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D. L. Lombardozzi et al.: Representing nighttime and minimum conductance in CLM4.5 325

Figure 2. Simulated average transpiration (a), runoff (d), and soil moisture (g) for a control simulation, and percent change from control in transpiration (bc), runoff (ef), and soil moisture (hi) after including a nighttime threshold ([Delta1]gnight; b, e, h) or a minimum gs threshold ([Delta1]gmin; c, f, i) based on observational data. Note that both nighttime and minimum thresholds were adjusted based on a soil moisture scalar.

have quantied gcut found that gcut was a low proportion, < 10 %, of total gs and less than measured gs,n (Caird et al., 2007; Zeppel et al., 2014). The values of go used in current implementations of the BallBerry model for C3 plants (10 mmol m2 s 1) fall within the range of measured gcut values (4 to 20 mmol m2 s1; Caird et al., 2007). Assuming go does have a theoretical function of representing gcut, rather than gs,n, incorporating an observed threshold of minimum gs is necessary. Whether go functions theoretically as gcut in the BWB model needs further evaluation, as adjusting simulated go has large impacts on canopy conductance and transpiration (Fig. 1; Barnard and Bauerle, 2013). Regardless, observed gs,n is larger than modeled go and functions differently, and therefore should be considered independently in model parameterizations.

The [Delta1]gmin and [Delta1]gnight simulations represent the intended change in minimum gs with greater delity, by limiting the minimum value without increasing gs at every model time step. Interestingly, in restricting only nighttime conductance, the [Delta1]gnight simulation allows daytime gs to decrease below the nighttime threshold during the dry season in semi-arid ecosystems (Fig. 1a). This occurs when An nears zero in

shade or low humidity, causing gs to fall to the default (lower) go. In contrast, the [Delta1]gmin simulation restricts minimum gs at all times, and therefore daytime values are never less than the water-adjusted gs,n. This increases canopy-averaged daytime gs, and hence transpiration, compared to the unmodied simulation whenever daytime gs values fall below the minimum threshold (Fig. 1a, c).

The data in Table S1 are a compilation of all available published gs,n data to date, and report gs,n values for 204 distinct plants. Of these, only four plants exhibit higher gs,n than daytime gs, and two of those are Crassulacean acid metabolism (CAM) plants, which by denition open their stomata at night to gain carbon dioxide and close their stomata during the day, and were not used in our parameterization. These data suggest that, as expected, gs,n is typically less than daytime gs. Most data presented in Table S1 are average values under non-drought stressed conditions, and are likely only reported for leaves in sunlit canopy layers. Thus, these data do not elucidate whether, at any given time, daytime values might drop below the nighttime threshold, but only suggest that, on average, they do not.

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326 D. L. Lombardozzi et al.: Representing nighttime and minimum conductance in CLM4.5

Figure 3. Average gross primary productivity (GPP) for a control simulation (a), and percent change from control (bc) after including a nighttime threshold ([Delta1]gnight; b) or a minimum gs threshold ([Delta1]gmin; c) based on observational data. Note that both nighttime and minimum thresholds were adjusted based on a soil moisture scalar.

In the context of the model simulations, low daytime gs oc-curs any time that Ahr/C is low. These are conditions which are poorly illuminated (in shade or at dawn/dusk and night), or when humidity is low. The CLM4.5SP contains a representation of the shaded canopy, which has lower gs and often reaches the minimum daytime threshold (go in the unmodied, [Delta1]go, and [Delta1]gnight simulations, and gs,n in the [Delta1]gmin simulation). The central issue in determining whether the [Delta1]gmin or [Delta1]gnight simulation is a better representation of minimum gs is whether, under the same conditions in the real world, daytime gs might be lower than gs,n. For example, if observational data support that daytime gs is less than gs,n in shaded canopy layers given the same water availability, then the [Delta1]gnight simulation is a better parameterization. However, if observational data suggest that daytime gs is consistently higher than gs,n, then the [Delta1]gmin simulation is a better parameterization. While observational data are not available to specically answer this question, the available data presented in Table S1 and data from Dawson et al. (2007), which suggest that gs,n is a fraction of daytime gs, imply that daytime gs is on average higher than gs,n, providing partial support for the [Delta1]gmin implementation. A different implementation of gs,n might calculate gs,n as a proportion of daytime gs, based on Dawson et al. (2007), who nd that gs,n is a proportion of daytime gs that changes based on days since last rainfall. We do not test this potential method here, but acknowledge it as a viable alternative to be considered.

The possible existence of a higher gs,n compared to daytime gs raises an interesting question about the potential selective advantage for leaves with a high gs,n. It is hypothesized that high gs,n may provide a benecial function to the plant, such as embolism repair or phloem transport (e.g., Dawson et al., 2007). Additionally, gs,n may contribute to xylem relling, potentially improving carbon gain by making water available when light conditions allow for photosynthesis (Dawson et al. 2007). Critically, it is not clear whether these potential functions are only relevant at night (and daytime gs can be lower than gs,n), or whether high gs,n is representative of a general strategy of higher overall minimum gs.

We are not aware of data that exist to support either possibility, and advocate for observations that will help determine the functional signicance of gs,n.

From a model or theoretical perspective, it is important to note that the reason that simulated gs values are reduced to as low as 10 mmol m2 s1 (or lower, if down-regulated for water stress) is a function of the universal parameterization of all C3 plants with that value of go. Given that it is unlikely that this value is universal for all plants, we consider that the large difference between the [Delta1]gmin or [Delta1]gnight simulations is an artifact of the poorly constrained parameterization of the daytime BWB model.

It should be noted that all the minimum thresholds implemented in our simulations ([Delta1]go, [Delta1]gnight, and [Delta1]gmin) are adjusted by a soil water scalar ( soil). Therefore, the nighttime ([Delta1]gnight) and the minimum ([Delta1]gmin) thresholds are altered according to the degree of soil moisture stress. When the daytime gs value is lower than the gnight threshold in the [Delta1]gnight simulation (Fig. 1c), the gnight threshold is already down-regulated for water stress. In this scenario, the daytime minimum gs is less than the nighttime gs when water stress is equivalent.

Responses to dry soil conditions are mediated both through the minimum gs values, and through the impact of soil moisture on photosynthetic capacity and leaf maintenance respiration, which are also multiplied by soil. Many of the impacts of our simulations result from feedbacks between higher transpiration rates resulting in faster depletion of soil moisture store, and therefore greater constraint on photosyn-thesis. These results are all emergent features of the model and should not be interpreted as direct results of the altered parameterization.

3.2 Global water and carbon

When averaged over 25 years, incorporating observed rates of gs,n in the [Delta1]gmin simulation increased transpiration losses by up to 30 % in the Amazon, and > 30 % in some arid regions, in part due to the small absolute magnitude of available soil water (Fig. 2ac). Semi-arid regions are primarily

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Figure 4. Terrestrial coupling for JuneJulyAugust for a control simulation (a), and the difference from control (bc) after including a nighttime threshold ([Delta1]gnight; b) or a minimum gs threshold value ([Delta1]gmin; c) based on observational data. Note that both nighttime and minimum thresholds were adjusted based on a soil moisture scalar.

Table 2. Global values from CLM simulations and observations .

Simulation gs,n GPP ET Runoff data used (Pg C yr1) (103 km3 yr1) (103 km3 yr1)

Control n/a 157.83 65.6148 30.462 go Mean 152.56 72.6555 24.2141 gnight Mean 156.068 66.0926 30.0724 gmin Mean 151.252 68.6843 27.8161 go Median 153.641 71.5441 25.1739 gnight Median 156.346 66.031 30.119 gmin Median 152.385 67.8881 28.51

Observation 119175 65.13 37.7521

Global gross primary productivity (GPP), evapotranspiration (ET), and runoff values. Observed values presented in Bonan et al. (2011), Welp et al. (2011), and Lawrence et al. (2011). n/a not applicable

broad-leaf shrub and C3 grass PFTs that have particularly high values (130 and 156 mmol m2 s1 respectively) of ob-served gs,n (Table 1), and have high nighttime vapor pressure decits that interact with higher minimum gs values, causing large nighttime transpiration rates. Using median rather than mean values caused only small (< 1.5 %) differences in global transpiration (Figs. S3, S4). Though the magnitude of response is different depending on parameterization used, the increases in transpiration imply that current model estimates of plant water loss are underestimated in many regions.

Simulated higher transpiration resulting from higher minimum gs also has ecosystem-scale ramications for hydrology (McLaughlin et al., 2007). For example, the increased transpiration resulted in drier soils compared to the control simulation (Fig. 2gi), with [Delta1]gmin causing > 40 % soil moisture decreases in semi-arid ecosystems like the southwestern United States and much of Australia (> 10 % in [Delta1]gnight). Additionally, the [Delta1]gmin-estimated changes to surface runoff are large in some regions, such as the 1025 % decreases in the tropics (510 % in [Delta1]gnight; Fig. 2df), suggesting that current runoff estimates may be too large. It should be noted that the difference between the [Delta1]gmin and [Delta1]gnight simulations is largely due to changes in minimum gs that affect daytime gs (see Sect. 3.1). Hydrologic changes in soil moisture and runoff in response to increased gs have previously

been documented in catchments in the southeastern United States (McLaughlin et al., 2007), and our results suggest that changes to stomatal conductance have similar consequences in CLM4.5SP simulations. Additionally, increasing minimum gs caused gross primary productivity (GPP) to decrease (Fig. 3) by 10 to > 25 % in many semi-arid regions.These are regions where water availability already restricts GPP, and the decreases in soil moisture caused by higher transpiration likely impart even more drought-induced stomatal closure.

To more directly evaluate the potential inuence of minimum gs on the climate system, we calculate the change in terrestrial coupling to the atmosphere. The terrestrial coupling index (Dirmeyer, 2011) estimates the degree to which changes in soil moisture control surface energy uxes to the atmosphere. This study uses root-zone soil moisture, rather than soil moisture over spatially constant soil depth, to highlight the direct impact of vegetation and minimum gs on surface uxes. Here we calculate the terrestrial coupling index during boreal summer months when warmer temperatures allow for the highest gs rates. We nd that the terrestrial coupling strength increases when using the [Delta1]gmin implementation, but is generally unchanged for [Delta1]gnight (Fig. 4), meaning root-zone soil moisture exerts a greater control on surface ux variability for [Delta1]gmin, largely due to the impact this sim-

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328 D. L. Lombardozzi et al.: Representing nighttime and minimum conductance in CLM4.5

Figure 5. Average diel canopy transpiration for the months of May, June, and July in Castlereagh, Australia (observation, dotted black line), estimated from sap ux measurements of Red Gum and Iron Bark, the dominant tree species in the canopy. Average simulated canopy transpiration for the grid cell corresponding to Castlereagh, Australia, for the control (unmodied; solid black line), [Delta1]go (BallBerry go parameter adjusted; red line), [Delta1]gnight (minimum nighttime threshold added; teal line), and [Delta1]gmin (minimum conductance threshold added;

blue line) simulations. Error bars corresponding to the observations (dashed) and each simulation (solid) are colored accordingly, and are

calculated as one standard deviation from the mean. Note that the simulations use meteorological forcings from an atmospheric dataset

(see Methods), not the local meteorology from when the measurements were collected (some meteorological data were collected at the site, but not all variables required by the model). The simulated grid cell covers a much larger area than the observational data collection site.

ulation has on daytime gs. This increased terrestrial coupling to the atmosphere largely mirrors the reductions in GPP and soil moisture in semi-arid ecosystems, and may reinforce climate extremes such as droughts or heat waves (Hirschi et al., 2011; Miralles et al., 2014).

3.3 Evaluating gs,n

Evaluating the performance of the new gs,n parameterizations is challenging for numerous reasons. First, our model scales from leaf-level gs and gs,n estimates to canopy transpiration. The best way of evaluating the model is to compare simulated and observed canopy transpiration because the model captures the average of an entire canopy, which is comprised of multiple plant functional types, rather than individual plant functional types. Incorporating realistic minimum gs increases global evapotranspiration and decreases global runoff compared to globally scaled observations, while estimates of GPP from all simulations fall within the range of global GPP estimates from observations (Table 2; Bonan et al., 2011, 2012; Li et al., 2011). However, these comparisons should be used with caution, since eddy covariance data used in estimating the GPP and evapotranspiration observations are susceptible to errors at night (Fisher et al., 2007; van Gorsel et al., 2008; Kirschbaum et al., 2007; Medlyn et al., 2005) due to a lack of sufcient canopy turbulence

that precludes detection of nighttime transpiration using this measurement methodology, and are not useful for evaluating the changes in water uxes tested in this study. Other data for evaluating model responses to minimum gs on large spatial scales are not yet available.

A comparison of simulated canopy transpiration to transpiration calculated from sap-ux data in Australia (Fig. 5) illustrates that a minimum gs threshold changes transpiration estimates during the early part of the night, though simulated nighttime rates are still low compared to observations. All model parameterizations fall within the observational range of uncertainty, but under-predict nighttime and midday canopy transpiration during May and June, and over-predict midday canopy transpiration in July. The lack of delity between the various model parameterizations and the observations is likely affected by the fact that observed meteorological data were unavailable to force the model. Therefore, key parameters driving both daytime and nighttime transpiration uxes, such as VPD and soil water availability, were likely different in the model simulations compared to the actual meteorological conditions at Castlereagh, Australia, during data collection. Additionally, because sap ow is measured at the base of the tree, there is typically a lag between when sap ow is measured and when the canopy transpires, and this lag is also notable in comparing observed sap ow with sim-

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D. L. Lombardozzi et al.: Representing nighttime and minimum conductance in CLM4.5 329

ulated estimates of transpiration. Estimating nighttime transpiration using sap-ow methodology is also convoluted with the relling of aboveground water stores depleted during the day, and thus is not directly comparable to our simulations.It should also be noted that the model does not have a semi-arid plant functional type, so semi-arid plants are typically represented in the model as deciduous plant functional types.

Given that our study focused only on one aspect of the gs formulation within a land-surface model, evaluating daytime gs and other aspects of the BWB model function (i.e., photosynthetic drivers of daytime gs, feedbacks to water availability, etc.) are all subject to pre-existing deciencies in the representation of a host of other model processes. For example, there are only two values of the g1 (slope) parameter in the BWB model, one for C3 and one for C4 plants (Sellers et al., 1996), and this parameter has not been modied or comprehensively evaluated within the context of the CLM4.5SP. Indeed, the use of the BWB model at all is currently the subject of some debate (Bonan et al., 2014; De Kauwe et al., 2015), and this study additionally highlights how the empirical nature of the BWB model leads to difculties when attempting to implement mechanistic processes.Further, daytime gs is also dependent on the photosynthetic capacity, and observations of Vcmax and Jmax (Bonan et al., 2011; Kattge and Knorr, 2007) indicate very wide ranges of plant functional type variation in these properties, also limiting our condence that the globally averaged parameters used in the default model will lead to accurate gs and transpiration at most locations. We choose not to focus on these and other parameters that effect daytime gs, as it does not directly impact the representation of gs,n, and is therefore beyond the scope of this paper.

4 Conclusions

The rate of minimum gs estimated from the BWB model used in many global land-surface models is typically smaller than observed gs,n (Barnard and Bauerle, 2013), as demonstrated in a review of 204 species (Zeppel et al., 2014). Including a nighttime or minimum gs threshold based on observations results in simulated hydrologic changes, such as decreased soil moisture and runoff (Fig. 2), particularly in semi-arid regions where water availability already restricts growth. In addition to potentially increasing drought stress in sensitive regions, this has the impact of reducing plant growth (Fig. 3) and changing the modeled terrestrial coupling to the atmosphere (Fig. 4). The difference between the [Delta1]gmin and [Delta1]gnight simulations highlights one outstanding uncertainty: does minimum daytime gs decrease below nighttime gs? While the balance of our arguments favors the [Delta1]gmin implementation of gs,n, this study primarily illustrates the potential sensitivity of global simulations to minimum gs considerations, and serves as motivation for additional eld experiments, particularly in semi-arid areas, to discern bet-

ter representations of low gs conditions during daytime and nighttime. To better understand the future of these sensitive ecosystems, widespread eld observations, quantication of minimum daytime gs, and a better understanding of the physiological causes and consequences of nighttime transpiration are necessary so that land-surface models can robustly incorporate observations and theory.

5 Code and data availability

The code for CLM4.5 is publicly available through a Subversion code repository: https://svn-ccsm-models.cgd.ucar.edu/cesm1/release_tags/cesm1_2_2

Web End =https://svn-ccsm-models. https://svn-ccsm-models.cgd.ucar.edu/cesm1/release_tags/cesm1_2_2

Web End =cgd.ucar.edu/cesm1/release_tags/cesm1_2_2 . To access the code, ll out a short, required registration to get a user name and password, necessary to gain access to the repository: http://www.cesm.ucar.edu/models/register/register_cesm.cgi

Web End =http://www.cesm.ucar.edu/models/register/ http://www.cesm.ucar.edu/models/register/register_cesm.cgi

Web End =register_cesm.cgi , http://www.cesm.ucar.edu/models/cesm1.2/clm/CLM45_Tech_Note.pdf

Web End =http://www.cesm.ucar.edu/models/cesm1. http://www.cesm.ucar.edu/models/cesm1.2/clm/CLM45_Tech_Note.pdf

Web End =2/clm/CLM45_Tech_Note.pdf . The CLM4.5 Users Guide can be found at http://www.cesm.ucar.edu/models/cesm1.2/clm/models/lnd/clm/doc/UsersGuide/book1.html

Web End =http://www.cesm.ucar.edu/models/cesm1.2/ clm/models/lnd/clm/doc/UsersGuide/book1.html. All stomatal conductance data used in developing the implementations can be found in Table S1. The modied code for CLM4.5 used in the go, gnight, and gmin simulations, as well as the data from the model simulations used in these analyses, are available upon request.

The Supplement related to this article is available online at http://dx.doi.org/10.5194/gmd-10-321-2017-supplement

Web End =doi:10.5194/gmd-10-321-2017-supplement .

Author contributions. D. L. Lombardozzi, M. J. B. Zeppel, andR. A. Fisher conceived the project. M. J. B. Zeppel assembled the gs,n datasets, D. L. Lombardozzi ran model simulations, and

D. L. Lombardozzi and A. Tawk analyzed model simulations, with guidance from R. A. Fisher. All authors contributed to writing the paper.

Acknowledgement. We thank Gordon Bonan for useful discussion on the manuscript, and the reviewers for the constructive comments that have improved the nal version of this paper. D. L.Lombardozzi was supported through the DEB Ecosystem Science Cluster and National Science Foundations grant EF-1048481.M. J. B. Zeppel was supported by ARC DECRA DE120100518.R. A. Fisher was supported by the National Science Foundation and the National Center for Atmospheric Research, and A. Tawk was supported by the National Science Foundation grant 0947837 for Earth System Modeling post-doctoral fellows. The National Center for Atmospheric Research is funded by the National Science Foundation.

Edited by: T. KatoReviewed by: J. B. Fisher, K. Tu, and one anonymous referee

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330 D. L. Lombardozzi et al.: Representing nighttime and minimum conductance in CLM4.5

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