I just read the paragraph about the Effectiveness of Rainfall in UNIT.2 Climate of Pakistan, p. 30.
The author bring the topic of the rain being ineffective in a context where the rain is expected for crops and farming, or in other words, on the potential usefulness of the said rain for crops, which is quite low considering the climatic context in this area. I do not think the author is trying to explain the actual process of evapotranspiration for the area.
This is a summary of the factors bring by the author on why the current pluvial regime on Pakistan is not 'effective':
- Rainfall is mainly from monsoon. Monsoon rain is not regular (and therefore unreliable for planning long term with rain as an only source of water). For instance ,distribution, amount and timing varies.
- High temperatures during the monsoon season imply high potential evapotranspiration so a greater proportion of the rainfall is evaporated rather than infiltrated to be available for crop growth
- Rain when it occurs is quite intense, with often a low infiltration rate and flow away instead of recharging ground moisture
- Insufficient rain during winter to compensate
Therefore, irrigation and other means to store and distribute water should be used in the long term to efficiently use the water input, in regard with the potential use for water.