| ET0 is one of the most important parameters for hydrological cycle and determine crop water demand. It was estimated using Penman-Motheith (PM) and FAO24 Penman (FP) approaches based on 135 weather stations distributed all over Inner Mongolia with 30 years observed data (1960-1990). The daily and yearly average ET0 was obtained and did research on the area distribution. The principal factors to influence ET0 in the climate of extreme arid, semiarid and relative humid were analyzed and filtraed.Wavelet theory was employed for scale analysis of ET0 using the data from three stations representing typical climate conditions, which are located at the west, the middle, and the east of Inner Mongolia. As a result of this, the errors of ET0 between PM and FP approaches were corrected based on the 30 years data from these stations.The technology of GIS was used to draw ET0 isogram with 30 years observed data and the distribution characteristics were analyzed by capacity of evaporation and transpiration in Inner Mongolia.To Based on 135 weather stations and 30 years meteorologic data for month-to-month, both the method of PM and FP were applied to caculate the value of ET0,ETrad and ETaero.The results as follow:(1). ET0 calculated by PM is greater than that by FP. The maximum difference is 0.76mm/d with a different rate of 15% and the minimum difference is 0.02mm/d with a different rate of 0.2%.(2). The evapotranspiration in Inner Mongolian area reduced gradually from west to east, the maximum value of ET0 by PM was 7.22mm/d(1148mm/y), that was lake Guaizi located in Ejin Qi at the most west, the minimum value was 2.94mm/d(538mm/y) at the east of Ergun Zuo Qi in the most Northeast area. The evapotranspiration in the mid-west area was big, which suit the characteristic of region climate.(3). The change of ETrad value computed by PM and FP were steady, but the ETaero value varied within wide limits. The sensitivity analysis shown that wind speed and air temperature were the most sensitive parameters for ETaero and ETrad respectively. ETrad and ETaero increased with the decrease of humidity, and vice versa. We chose daily meteorologic data representing typical climate conditions, aquired the value of ET0,ETrad and ETaero by PM and carried key analysis to ET0 and meteorological factors that had influence on the three value:(1). The whole tendency of ET0 value was sorted by Ejin Qi > Hohhot >Ergun Zuo Qi(2). Th change tendency of ETaero and ET0 was basic the same in extreme arid area (Ejin Qi), where the ET0 was mainly influenced by aerodynamic force and the change of ETrad was steady.(3). The range of ET0 falled steadily in the semi-arid region(Huhhot), where the ET0 was mainly influenced by radiation and aerodynamic force. the change scope of ETaero and ETrad was greater.(4). Th change tendency of ETaero and ET0 was basic the same in semi-humid area(Ergun Zuo Qi), where the ET0 was mainly influenced by radiation.The change of ETaero was relative steady ,but the change scope of ETrad was greater.According to wavelet theory for scale analysis of ET0 ,the change of ET0 and its constitutional items—radiation and aerodynamic force were mainly experienced 3 stages from April to September. The change in the 3 areas laged gradually. Moreover, the change of radiation slightly advanced ET0 for each aera, while the change of aerodynamic force laged behind ET0. At the same time, the point of inflection also obeyed the similar rule. In the entire time domain, ET0 and its constitutional items—radiation and aerodynamic force at the 3 areas had relative good corresponding relationship.(1). The change characteristic of ET0,ETrad and ETaero at the three areas: From mid-May to the end of July ET0,ETrad and the ETaero were all greater at west, the ETrad was greater from the beginning of June to the end of August ,which slightly lagged behind the change of ET0 for 10 days. From April, ET0 and its constitutional item—radiation at the middle part experienced 3 stages, namely before May (the few times), to the end of August (many times), to September (few times). The aerodynamic force also experienced 3 change stages, but its change would be in advance one month.(2). The characteristic and rule of multiple time scale at the three areas: For the 45days'time scale, the western area mainly experienced two stages of subrogation: the beginning of May and mid-August were the point of inflection. Before this point, the ET0 was at bigger period, after this point it was at smaller period. The point of inflection of ETrad relativly lagged (at the end of May, the beginning of September).The distribution of ET0,ETrad and ETaero in various time scale of periodic change had the difference at middle area, where the vibrational period varied at the different period ,but the strongest vibrating centre was at the beginning of July. Compared with the western area, the point of inflection laged slightly.The distribution of ET0,ETrad and ETaero in various time scale of periodic change had the difference at eastern area, where the vibrational period varied at the different period ,but the strongest vibrating centre was at the beginning of July. However, the vibration of was relative complicated, the vibrating centre of 20 days located at the middle of May and the beginning of June for 10 days. Compared with the middle area, the point of inflection laged slightly.(3). The strongest vibration of ET0,ETrad and ETaero respectively were about 5days and 10 days at Ejin Qi, 45days and 15 days at Hohhot. At Ergun Zuo Qi , the strongest vibration of ET0 and ETrad were about 60days , strongest vibration of ETaero was about 25 days.Drew the isogram of ET0 using GIS software:(1). Not only could satisfy the low-accuracy request at initial stage by the layout of planting area and the program of irrigation system, but also could meet the high-precision application of request for real-time irrigation system.(2). It had great significance in reflecting the spatial distribution of ET0 in Inner Mongolia and serving for the productive practice more direct-viewing, higher efficiency and higher accuracy.
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