Drips from Leaf to Leaf

Subroutine Drips (cuintc.f)

"The water intercepted by the leaf can (1) evaporate into and humidify the canopy air space, (2) drip from the leaf to other leaves or the ground, (3) reside on the leaf or some other plant part (such as the cup formed by the attachment of the leaf sheath to the stock on corn), or (4) run down the stem to the soil and be evaporated or infiltrated there.

Significant amounts of water will not drip from a leaf or run down the stem until some maximum amount of water resides on that leaf. ... This intercepted water in not uniformly distributed over the leaf surface; it 'beads up' and has been observed to wet from 10-50% of the leaf surface. A typical wetted leaf area fraction (where transpiration is assumed to be zero and only droplet evaporation occurs) is about 0.20. the remaining 80% of the leaf area is assumed to be transpiring." (Norman & Campbell 1983, p 161)

• Calculate WTP0(J) - a weighting factor for each layer corresponding to the fraction of precipitation intercepted by layer J, assuming drips are falling straight down. [50]

• Compare the amount of intercepted precip (PINT(J)) to the amount evaporated (EVIMM(J)) from that layer:
  • If PINT(J) < EVIMM(J), there are no drips and the leaves are drying over this timestep (the leaf balance subroutine will have to be run again).
  • If PINT(J) > EVIMM(J), water accumulates on the plants.
    • If Pnet (=PINT-EVIMM) < Pstorage (=PINTMX*2*DF), the accummulated water just sits in the current layer, losing the evaporated amount. [75]
    • If Pnet > Pstorage,
      • a certain amount of the excess drips from the leaves to lower layers DRIP(J) = (1-FRSTEM)*(Pnet-Pstorage),
      • and the rest of the excess runs down the stem (STEM).
      • The drips get traced down through the lower layers, losing a fraction WTP0 to each layer. [90]

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