Soil-Water-Plant Relationship

Philip's two-term infiltration equation expresses cumulative infiltration depth as $I = S \cdot t^{1/2} + A \cdot t$, where sorptivity $S = 24.0\text{ mm/h}^{1/2}$ and transmissivity factor $A = 5.0\text{ mm/h}$. The infiltration rate $i = \frac{dI}{dt}$ at elapsed time $t = 4.0\text{ hours}$ in millimeters per hour ($\text{mm/h}$) is ________ (round off to one decimal place).