Comparison of nighttime with daytime evapotranspiration responses to environmental controls across temporal scales along a climate gradient
Authors: Han Qiong , Wang Tiejun , Wang Lichun , Smettem Keith , Mai Mai , Chen Xi 
Published: March 2022

&lt;p&gt;Understanding daytime (&lt;em&gt;ET&lt;/em&gt;&lt;em&gt;&lt;sub&gt;D&lt;/sub&gt;&lt;/em&gt;)&amp;#160;and nighttime (&lt;em&gt;ET&lt;/em&gt;&lt;em&gt;&lt;sub&gt;N&lt;/sub&gt;&lt;/em&gt;) evapotranspiration is critical for accurately evaluating terrestrial water and carbon cycles. However, unlike &lt;em&gt;ET&lt;/em&gt;&lt;em&gt;&lt;sub&gt;D&lt;/sub&gt;&lt;/em&gt;, the factors influencing &lt;em&gt;ET&lt;/em&gt;&lt;em&gt;&lt;sub&gt;N&lt;/sub&gt;&lt;/em&gt;&amp;#160;remain poorly understood. Here, long-term &lt;em&gt;ET&lt;/em&gt;&lt;em&gt;&lt;sub&gt;D&lt;/sub&gt;&lt;/em&gt;&amp;#160;and &lt;em&gt;ET&lt;/em&gt;&lt;em&gt;&lt;sub&gt;N&lt;/sub&gt;&lt;/em&gt; data from five FLUXNET sites along a climate gradient in Northern Australia were analyzed to compare their responses to environmental drivers at different temporal scales. Across the sites, mean annual&lt;em&gt;&amp;#160;ET&lt;/em&gt;&lt;em&gt;&lt;sub&gt;N&lt;/sub&gt;&lt;/em&gt;/&lt;em&gt;ET&lt;/em&gt;&lt;em&gt;&lt;sub&gt;D&lt;/sub&gt;&lt;/em&gt;&amp;#160;ranged between 5.1% and 11.7%, which was mainly determined by &lt;em&gt;ET&lt;/em&gt;&lt;em&gt;&lt;sub&gt;D&lt;/sub&gt;&lt;/em&gt; variations. Both vegetation and climatic conditions were closely related to mean annual &lt;em&gt;ET&lt;/em&gt;&lt;em&gt;&lt;sub&gt;D&lt;/sub&gt;&lt;/em&gt;, while the primary controls on mean annual &lt;em&gt;ET&lt;/em&gt;&lt;em&gt;&lt;sub&gt;N&lt;/sub&gt;&lt;/em&gt; were air temperature and net radiation (&lt;em&gt;Rn&lt;/em&gt;). At site levels, monthly &lt;em&gt;ET&lt;/em&gt;&lt;em&gt;&lt;sub&gt;D&lt;/sub&gt;&lt;/em&gt;&lt;em&gt;&amp;#160;&lt;/em&gt;and &lt;em&gt;ET&lt;/em&gt;&lt;em&gt;&lt;sub&gt;N&lt;/sub&gt;&lt;/em&gt;&amp;#160;showed better correlations with meteorological and vegetation&amp;#160;variables than annual &lt;em&gt;ET&lt;/em&gt;&lt;em&gt;&lt;sub&gt;D&lt;/sub&gt;&lt;/em&gt;&lt;em&gt;&amp;#160;&lt;/em&gt;and &lt;em&gt;ET&lt;/em&gt;&lt;em&gt;&lt;sub&gt;N&lt;/sub&gt;&lt;/em&gt;, and the coupling of &lt;em&gt;ET&lt;/em&gt;&lt;em&gt;&lt;sub&gt;D&lt;/sub&gt;&lt;/em&gt;&lt;em&gt;&amp;#160;&lt;/em&gt;and &lt;em&gt;ET&lt;/em&gt;&lt;em&gt;&lt;sub&gt;N&lt;/sub&gt;&lt;/em&gt;&amp;#160;was also stronger at monthly timescales, particularly under drier climatic conditions.&amp;#160;At daily timescales, leaf area index and soil water content (&lt;em&gt;SWC&lt;/em&gt;) controlled &lt;em&gt;ET&lt;/em&gt;&lt;em&gt;&lt;sub&gt;D&lt;/sub&gt;&lt;/em&gt;&amp;#160;with&lt;em&gt;&amp;#160;SWC&lt;/em&gt;&amp;#160;being more important at drier sites; whereas, &lt;em&gt;SWC&lt;/em&gt;&amp;#160;was the dominant factor controlling &lt;em&gt;ET&lt;/em&gt;&lt;em&gt;&lt;sub&gt;N&lt;/sub&gt;&lt;/em&gt;.&amp;#160;At half-hourly timescales, the boosted regression tree method&amp;#160;quantitively showed that &lt;em&gt;ET&lt;/em&gt;&lt;em&gt;&lt;sub&gt;D&lt;/sub&gt;&lt;/em&gt;&amp;#160;and &lt;em&gt;ET&lt;/em&gt;&lt;em&gt;&lt;sub&gt;N&lt;/sub&gt;&lt;/em&gt;&amp;#160;were controlled by&lt;em&gt;&amp;#160;Rn&lt;/em&gt;&amp;#160;and &lt;em&gt;SWC&lt;/em&gt;, respectively.&amp;#160;Overall, the results showed that &lt;em&gt;ET&lt;/em&gt;&lt;em&gt;&lt;sub&gt;N&lt;/sub&gt;&lt;/em&gt;&amp;#160;was less responsive to environmental variables, illustrating&amp;#160;that &lt;em&gt;ET&lt;/em&gt;&lt;em&gt;&lt;sub&gt;D&lt;/sub&gt;&lt;/em&gt;&amp;#160;and &lt;em&gt;ET&lt;/em&gt;&lt;em&gt;&lt;sub&gt;N&lt;/sub&gt;&lt;/em&gt;&amp;#160;responded differently to diverse climate regimes and ecosystems at varying temporal scales.&lt;/p&gt;