Sector sensitivity to infrastructure investments is critical in ascertaining the threshold investments towards achievement of sector objectives. The study investigates the relationship between water sector public investments and sector contribution to gross domestic product (GDP), water coverage and irrigation. Vector error correction mechanism and vector autoregression are applied on time series data spanning the period 1980-2016. The results indicate that Kenya requires approximately seven times more of development expenditure as initial drift followed by sustained medium term increment of over 10 percent to achieve and sustain 10 percent growth in both GDP contribution and water coverage, together with 14 percent increase in irrigation tonnage. Water investments take about 3 to 4 years to register significant positive results, showing that Kenya may need at least five 5-year medium term cycles to realize universal access to water and sustain growth in real GDP and irrigation productivity.