The irrigation sector accounts for over 70% of the total freshwater consumptionin the world. Therefore, e cient management of irrigation water is essential toensure water, food, energy and environmental securities in a sustainable manner;these securities are grand challenges of the 21st century. The main objective of thisresearch is to evaluate the simulation of irrigation water demand at the catchmentscale in order to develop improved tools for conducting quantitative planning andclimate change studies. Irrigation water demand is mostly driven by soil moisture.It is a state variable which is used to trigger the irrigation in hydrological models.In this study, a hydrolgical model (Soil and Water Assessment Tool, SWAT) isevaluated for reliably simulating the spatial and temporal patterns of soil moistureat a catchment scale. The SWAT simulated soil moisture was compared with theindirect estimates of soil moisture from Landsat and Time-domain reectometry(TDR). The results showed that the SWAT simulated soil moisture was comparablewith the soil moisture estimated from Landsat and TDR.Secondly, the applicability of the SWAT model was tested for simulatingstreamow, evapotranspiration (ET) and irrigation water demand for four di erentagro-climatic zones (Mediterranean, Subtropical monsoon, Humid, and Tropical).Two di erent irrigation scheduling techniques were used to simulate irrigationnamely, soil water de cit and plant water demand. It was seen from the resultsthat the SWAT simulated irrigation amounts under soil moisture irrigationscheduling technique were close to the irrigation statistics provided by the state.However, the irrigation amounts simulated under the plant water demandirrigation scheduling technique were underestimated. Additionally, the tworeanalysis data were also used to check the data uncertainty in simulatingirrigation water demand.SWAT model code was modi ed by incorporating modi ed root densitydistribution function and dynamic stress factor. The modi ed model was used tosimulate irrigation and crop yield. It was tested against the irrigation and cropyield simulated by Soil Water Atmosphere Plant (SWAP) model and eld data(Hamerstorf, Lower Saxony, Germany). It was then validated for di erentcatchments (Germany, India and Vietnam). The results showed that the SWATsimulated irrigation water demand in case of plant water demand is comparablewith the amount simulated by the model under soil water de cit irrigationscheduling technique.This dissertation not only bridges the gap between the scales of soil moisturedetermination but also establishes a close connection with the actual observationsand modelled soil moisture and irrigation amounts at the eld, regional and globalstudies in agricultural water management. Additionally, the studies aboutsimulating irrigation water requirement in data-scarce areas must address datauncertainty when using reanalysis data. It was found that rainfall is not always the dominant variable in irrigation simulation. Therefore, it is worth checking andbias correct the other climate variables.