Soil-Water-Plant Relationship

In Kostiakov's cumulative infiltration equation $Z = k t^a$, cumulative infiltration depth is $Z = 3.0\text{ cm}$ at $t = 1\text{ hour}$ ($k = 3.0$). If $a = 0.50$, calculate the cumulative infiltration depth in $\text{cm}$ after $t = 4\text{ hours}$.