### 2.20.4.1. 14.4.1 Stress-Deciduous Onset Triggers[¶](#stress-deciduous-onset-triggers "Permalink to this headline")

In climates that are warm year-round, onset triggering depends on soil water availability. At the beginning of a dormant period (end of previous offset period), an accumulated soil water index (\\({SWI}\_{sum}\\), d) is initialized (\\({SWI}\_{sum} = 0\\)), with subsequent accumulation calculated as:

(2.20.65)[¶](#equation-zeqnnum503826 "Permalink to this equation")\\\[\\begin{split}SWI\_{sum}^{n} =\\left\\{\\begin{array}{l} {SWI\_{sum}^{n-1} +f\_{day} \\qquad {\\rm for\\; }\\Psi \_{s,3} \\ge \\Psi \_{onset} } \\\\ {SWI\_{sum}^{n-1} \\qquad \\qquad {\\rm for\\; }\\Psi \_{s,3} <\\Psi \_{onset} } \\end{array}\\right.\\end{split}\\\]

where \\(\\Psi\\)s,3 is the soil water potential (MPa) in the third soil layer and \\({\\Psi}\_{onset} = -0.6 MPa\\) is the onset soil water potential threshold. Onset triggering is possible once \\({SWI}\_{sum} > 15\\). To avoid spurious onset triggering due to soil moisture in the third soil layer exceeding the threshold due only to soil water suction of water from deeper in the soil column, an additional precipitation trigger is included which requires at least 20 mm of rain over the previous 10 days [(Dahlin et al., 2015)](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/References/CLM50_Tech_Note_References.html#dahlinetal2015). If the cold climate growing degree-day accumulator is not active at the time when the soil moisture and precipitation thresholds are reached (see below), and if the daylength is greater than 6 hours, then onset is triggered. Except as noted below, \\({SWI}\_{sum}\\) continues to accumulate according to Eq. [(2.20.65)](#equation-zeqnnum503826) during the dormant period if the daylength criterion prevents onset triggering, and onset is then triggered at the timestep when daylength exceeds 6 hours.

In climates with a cold season, onset triggering depends on both accumulated soil temperature summation and adequate soil moisture. At the beginning of a dormant period a freezing day accumulator (\\({FD}\_{sum}\\), d) is initialized (\\({FD}\_{sum} = 0\\)), with subsequent accumulation calculated as:

(2.20.66)[¶](#equation-20-66 "Permalink to this equation")\\\[\\begin{split}FD\_{sum}^{n} =\\left\\{\\begin{array}{l} {FD\_{sum}^{n-1} +f\_{day} \\qquad {\\rm for\\; }T\_{s,3} >TKFRZ} \\\\ {FD\_{sum}^{n-1} \\qquad \\qquad {\\rm for\\; }T\_{s,3} \\le TKFRZ} \\end{array}\\right. .\\end{split}\\\]

If \\({FD}\_{sum} > 15\\) during the dormant period, then a cold-climate onset triggering criterion is introduced, following exactly the growing degree-day summation (\\({GDD}\_{sum}\\)) logic of Eqs. [(2.20.46)](#equation-zeqnnum510730) and [(2.20.47)](#equation-zeqnnum598907). At that time \\({SWI}\_{sum}\\) is reset (\\({SWI}\_{sum} = 0\\)). Onset triggering under these conditions depends on meeting all three of the following criteria: \\({SWI}\_{sum} > 15\\), \\({GDD}\_{sum} > {GDD}\_{sum\\\_crit}\\), and daylength greater than 6 hrs.

The following control variables are set when a new onset growth period is initiated: \\({SWI}\_{sum} = 0\\), \\({FD}\_{sum} = 0\\), \\({GDD}\_{sum} = 0\\), \\({n}\_{days\\\_active} = 0\\), and \\(t\_{onset} = 86400\\cdot n\_{days\\\_ on}\\), where \\({n}\_{days\\\_on}\\) is set to a constant value of 30 days. Fluxes from storage into transfer pools occur in the timestep when a new onset growth period is initiated, and are handled identically to Eqs. [(2.20.50)](#equation-zeqnnum904388) -[(2.20.56)](#equation-zeqnnum195642) for carbon fluxes, and to Eqs. [(2.20.57)](#equation-zeqnnum812152) - [(2.20.62)](#equation-zeqnnum605338) for nitrogen fluxes. The onset counter is decremented on each time step after initiation of the onset period, until it reaches zero, signaling the end of the onset period:

(2.20.67)[¶](#equation-20-67 "Permalink to this equation")\\\[t\_{onfset}^{n} =t\_{onfset}^{n-1} -\\Delta t\\\]

