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-<p>The global distribution of the moisture index (<a href="#fig-ai_map">Figure&nbsp;<span>8.22</span></a>) resembles global distributions of vegetation patterns (<a href="biogeography.html#sec-global-veg-patterns"><span>Section&nbsp;2.5</span></a>). In regions where radiation (and temperature) does not limit vegetation, it may additionally be limited by water, as reflected by the moisture index. When combining the data for the satellite remote-sensing based fAPAR and the moisture index, a similar pattern emerges as described in <a href="#sec-waterbucket-relations"><span>Section&nbsp;8.5</span></a>. The green vegetation cover, measured by fAPAR, initially increases linearly with increasing <em>P</em>/PET for low values of the latter. Beyond a certain value, fAPAR no longer shows a relationship with <em>P</em>/PET (<a href="#fig-donohue">Figure&nbsp;<span>8.23</span></a>).</p>
-<p>The slope of the initial linear increase of fAPAR vs.&nbsp;<em>P</em>/PET reflects the water-carbon coupling. The amount of active green and transpiring foliage area is limited by water availability. Under rising CO<sub>2</sub>, stomatal conductance tends to be reduced (<a href="gpp.html#fig-constant-chi">Figure&nbsp;<span>4.11</span></a>) and the water use efficiency is enhanced (<a href="gpp.html#sec-wue"><span>Section&nbsp;4.4.1</span></a>). In other word, a larger area of green leaves per unit ground area can be sustained for a given level of aridity. Indeed, research has shown that the relationship shown in <a href="#fig-donohue">Figure&nbsp;<span>8.23</span></a> is shifting such that the slope of the initial increase tends to steepen over time <span class="citation" data-cites="donohue13grl ukkola16natcc">(<a href="references.html#ref-donohue13grl" role="doc-biblioref">Donohue et al. 2013</a>; <a href="references.html#ref-ukkola16natcc" role="doc-biblioref">Ukkola et al. 2016</a>)</span>.</p>
+<p>The global distribution of the moisture index (<a href="#fig-ai_map">Figure&nbsp;<span>8.22</span></a>) resembles global distributions of vegetation patterns (<a href="biogeography.html#sec-global-veg-patterns"><span>Section&nbsp;2.5</span></a>). In regions where radiation (and temperature) does not limit vegetation, it may additionally be limited by water, as reflected by the moisture index and its low values across the worlds drylands.</p>
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+<p>When combining the data for the satellite remote-sensing based fAPAR and the moisture index, a similar pattern emerges as described in <a href="#sec-waterbucket-relations"><span>Section&nbsp;8.5</span></a>. The green vegetation cover, measured by fAPAR, initially increases linearly with increasing <em>P</em>/PET for low values of the latter. Beyond a certain value, fAPAR no longer shows a relationship with <em>P</em>/PET (<a href="#fig-donohue">Figure&nbsp;<span>8.23</span></a>).</p>
+<p>The slope of the initial linear increase of fAPAR vs.&nbsp;<em>P</em>/PET reflects the water-carbon coupling. The amount of active green and transpiring foliage area is limited by water availability. research has shown that the relationship shown in <a href="#fig-donohue">Figure&nbsp;<span>8.23</span></a> is shifting such that the slope of the initial increase tends to steepen over time <span class="citation" data-cites="donohue13grl ukkola16natcc">(<a href="references.html#ref-donohue13grl" role="doc-biblioref">Donohue et al. 2013</a>; <a href="references.html#ref-ukkola16natcc" role="doc-biblioref">Ukkola et al. 2016</a>)</span>. This is related to the fact that under rising CO<sub>2</sub>, stomatal conductance tends to be reduced (<a href="gpp.html#fig-constant-chi">Figure&nbsp;<span>4.11</span></a>) and the water use efficiency is enhanced (<a href="gpp.html#sec-wue"><span>Section&nbsp;4.4.1</span></a>). In other words, a larger area of green leaves per unit ground area can be sustained for a given level of aridity.</p>
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@@ -298,7 +298,7 @@
     "href": "ecohydrology.html#sec-budyko",
     "title": "8  Ecohydrology",
     "section": "8.7 Energy and water limitation across the globe",
-    "text": "8.7 Energy and water limitation across the globe\nEcosystems are commonly distinguished into energy-limited and water-limited systems. This notion relates to the dominant limiting resource and to the relations described in Section 8.5. When the bucket is full, the system is energy-limited. When it gets depleted, the system becomes water-limited. Of course, this is a simplification. The relations described in Section 8.5 really refer to a spectrum, rather than a binary classification.\nFurthermore, water-limited conditions may be temporally limited and interspersed by energy-limited periods. Also, the water bucket model, as formulated above, suggests that water-limitation sets in at the point when soil moisture falls below the field capacity. However, as long as the net radiation and the atmospheric vapour pressure deficit are not “excessive”, the demand for transpiration (Equation 8.7) may be met by the supply (Equation 8.8) without substantial stomatal closure during the daytime.\nA common classification of ecosystems into aridity classes is given in Table 8.1 based on Middleton and Thomas (1992). This considers the moisture index as defined by P/PET.\n\n\nTable 8.1: Aridity classes after Middleton and Thomas (1992).\n\n\nMoisture index value\nAridity class\n\n\n\n\n<0.03\nHyper arid\n\n\n0.03-0.2\nArid\n\n\n0.2-0.5\nSemi-arid\n\n\n0.5-0.65\nDry sub-humid\n\n\n>0.65\nHumid\n\n\n\n\nThe global distribution of the moisture index (Figure 8.22) resembles global distributions of vegetation patterns (Section 2.5). In regions where radiation (and temperature) does not limit vegetation, it may additionally be limited by water, as reflected by the moisture index. When combining the data for the satellite remote-sensing based fAPAR and the moisture index, a similar pattern emerges as described in Section 8.5. The green vegetation cover, measured by fAPAR, initially increases linearly with increasing P/PET for low values of the latter. Beyond a certain value, fAPAR no longer shows a relationship with P/PET (Figure 8.23).\nThe slope of the initial linear increase of fAPAR vs. P/PET reflects the water-carbon coupling. The amount of active green and transpiring foliage area is limited by water availability. Under rising CO2, stomatal conductance tends to be reduced (Figure 4.11) and the water use efficiency is enhanced (Section 4.4.1). In other word, a larger area of green leaves per unit ground area can be sustained for a given level of aridity. Indeed, research has shown that the relationship shown in Figure 8.23 is shifting such that the slope of the initial increase tends to steepen over time (Donohue et al. 2013; Ukkola et al. 2016).\n\n\n\n\n\nFigure 8.22: Moisture index defined as P/PET. This is also commonly referred to as the aridity index (but note: high values mean moist conditions). Data from Zomer, Xu, and Trabucco (2022).\n\n\n\n\n\n\n\n\n\nFigure 8.23: Vegetation greenness (measured by the fraction of absorbed photosynthetically active radiation, fAPAR) versus the moisture index (measured by the ratio of precipitation over potential evapotranspiration, P/PET). Data for fAPAR is from the MODIS MOD15A2 C006 product and taken as the annual maximum of mean monthly values. The color of hexagons shows the density of points (count per bin). A Data for the moisture index is from Zomer, Xu, and Trabucco (2022).\n\n\n\n\nRelated patterns emerge from the annual sums of ecosystem water fluxes (AET, PET, and P) measured from surface-atmosphere exchange (eddy covariance technique, Figure 8.24). However, at the scale at which ecosystem fluxes are measured (~1 km2), AET is sometimes larger than precipitation, as suggested by Figure 8.24 (b). This may be related to errors in precipitation and latent heat flux estimates, or due to vegetation having access to the groundwater and/or being supplied moisture through lateral subsurface flow.\n\n\n\n\n\nFigure 8.24: (a) Ecosystems in the AET vs. PET space. (b) Ecosystems in the Bukyko space (AET/P vs. PET/P). Each point represents a site from which flux measurements are available. A selection of sites from which data were used in this and previous chapters are highlighted. AET, PET, and P are multi-year means of annual sums."
+    "text": "8.7 Energy and water limitation across the globe\nEcosystems are commonly distinguished into energy-limited and water-limited systems. This notion relates to the dominant limiting resource and to the relations described in Section 8.5. When the bucket is full, the system is energy-limited. When it gets depleted, the system becomes water-limited. Of course, this is a simplification. The relations described in Section 8.5 really refer to a spectrum, rather than a binary classification.\nFurthermore, water-limited conditions may be temporally limited and interspersed by energy-limited periods. Also, the water bucket model, as formulated above, suggests that water-limitation sets in at the point when soil moisture falls below the field capacity. However, as long as the net radiation and the atmospheric vapour pressure deficit are not “excessive”, the demand for transpiration (Equation 8.7) may be met by the supply (Equation 8.8) without substantial stomatal closure during the daytime.\nA common classification of ecosystems into aridity classes is given in Table 8.1 based on Middleton and Thomas (1992). This considers the moisture index as defined by P/PET.\n\n\nTable 8.1: Aridity classes after Middleton and Thomas (1992).\n\n\nMoisture index value\nAridity class\n\n\n\n\n<0.03\nHyper arid\n\n\n0.03-0.2\nArid\n\n\n0.2-0.5\nSemi-arid\n\n\n0.5-0.65\nDry sub-humid\n\n\n>0.65\nHumid\n\n\n\n\nThe global distribution of the moisture index (Figure 8.22) resembles global distributions of vegetation patterns (Section 2.5). In regions where radiation (and temperature) does not limit vegetation, it may additionally be limited by water, as reflected by the moisture index and its low values across the worlds drylands.\n\n\n\n\n\nFigure 8.22: Moisture index defined as P/PET. This is also commonly referred to as the aridity index (but note: high values mean moist conditions). Data from Zomer, Xu, and Trabucco (2022).\n\n\n\n\nWhen combining the data for the satellite remote-sensing based fAPAR and the moisture index, a similar pattern emerges as described in Section 8.5. The green vegetation cover, measured by fAPAR, initially increases linearly with increasing P/PET for low values of the latter. Beyond a certain value, fAPAR no longer shows a relationship with P/PET (Figure 8.23).\nThe slope of the initial linear increase of fAPAR vs. P/PET reflects the water-carbon coupling. The amount of active green and transpiring foliage area is limited by water availability. research has shown that the relationship shown in Figure 8.23 is shifting such that the slope of the initial increase tends to steepen over time (Donohue et al. 2013; Ukkola et al. 2016). This is related to the fact that under rising CO2, stomatal conductance tends to be reduced (Figure 4.11) and the water use efficiency is enhanced (Section 4.4.1). In other words, a larger area of green leaves per unit ground area can be sustained for a given level of aridity.\n\n\n\n\n\nFigure 8.23: Vegetation greenness (measured by the fraction of absorbed photosynthetically active radiation, fAPAR) versus the moisture index (measured by the ratio of precipitation over potential evapotranspiration, P/PET). Data for fAPAR is from the MODIS MOD15A2 C006 product and taken as the annual maximum of mean monthly values. The color of hexagons shows the density of points (count per bin). A Data for the moisture index is from Zomer, Xu, and Trabucco (2022).\n\n\n\n\nRelated patterns emerge from the annual sums of ecosystem water fluxes (AET, PET, and P) measured from surface-atmosphere exchange (eddy covariance technique, Figure 8.24). However, at the scale at which ecosystem fluxes are measured (~1 km2), AET is sometimes larger than precipitation, as suggested by Figure 8.24 (b). This may be related to errors in precipitation and latent heat flux estimates, or due to vegetation having access to the groundwater and/or being supplied moisture through lateral subsurface flow.\n\n\n\n\n\nFigure 8.24: (a) Ecosystems in the AET vs. PET space. (b) Ecosystems in the Bukyko space (AET/P vs. PET/P). Each point represents a site from which flux measurements are available. A selection of sites from which data were used in this and previous chapters are highlighted. AET, PET, and P are multi-year means of annual sums."
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