The complete freezing of the upper 30 cm of water in a river would have several effects on the area's water cycle. Let's analyze the options provided:
A. There would be more water runoff entering the river.
This could be possible if the frozen surface prevents infiltration into the ground, leading to increased runoff during subsequent precipitation events. However, it's not a direct effect of freezing but rather related to changes in precipitation.
B. There would be no water evaporation leaving the river.
This option is likely true. When the surface of the river is frozen, evaporation would be significantly reduced because the ice prevents water from directly leaving the surface into the atmosphere.
C. There would be more precipitation in the area due to excessive condensation.
While the freezing water could lead to localized changes in temperature and humidity, it's not accurate to say it would cause excessive condensation leading to more precipitation in the area. Precipitation is influenced by larger weather patterns rather than localized freezing.
D. There would be no change in the area’s water cycle.
This is incorrect, as the freezing of the river would indeed affect evaporation rates and potentially impact other factors in the water cycle.
Based on this analysis, B. There would be no water evaporation leaving the river is the most accurate response to the question regarding the effects of complete freezing on the water cycle in the area.