Hydrological Simulation Of A Rainfed Minor Irrigation Tank

Hydrological Simulation Of A Rainfed Minor Irrigation Tank

Dr. Krishna Mohan M

Professor of Civil Engineering, Malla Reddy Engineering College (affiliated to JNTUH), Secunderabad

Abstract

Minor irrigation schemes play an important role in the rural livelihoods and economy of Andhra Pradesh state. The state has 12,351 MI tanks with an individual ayacut exceeding 40 ha commanding a total ayacut of

12.5 lakh hectares. The majority of these tanks are non-system fed tanks receiving inflows entirely from rainfall. In this paper it is attempted to study the assessment of physical benefits of one such non-system fed tank viz. Chittivalasa MI tank located in Bheemunipatnam mandal in Visakhapatnam district of Andhra Pradesh, India through hydrological simulation for 30 years. Stranges runoff model is used to compute inflows in to the tank. Modified penman method is used to compute crop water requirements. Such studies become necessary for the design of new MI tanks or for taking up rehabilitation of existing MI tanks for carrying out techno-economic feasibility to examine whether sufficient inflows are available from the upstream catchment areas for the prevailing spatial and temporal distribution of rainfall.

Keywords – Minor irrigation tanks, Hydrological Simulation, Assessment of physical benefits, Stranges runoff model, Modified penman method for computing crop water requirement

1. Introduction

   Minor irrigation schemes have been the backbone of agriculture in Andhra Pradesh as is the case with India as a whole. The importance of these schemes in the Indian agriculture sector was highlighted by the First Irrigation Commission (1901-03) and the Royal commission of Agriculture (1928). The crucial role that minor irrigation could play in augmenting food production with in a short time was specially recognized in the Growmore food campaign launched in 1943.

   The planning commission, since its inception, has been stressing the importance of minor irrigation schemes in increasing food production. Page 251 of first five year plan says that they (minor irrigation schemes) provide large amount of dispersal employment. They involve smaller outlay and can be executed in a comparatively shorter period. Being spread all over the country they confer widespread benefit and it is therefore easier to mobilize public cooperation in their construction. The food grain enquiry committee (1952) also reiterated the need for paying greater attention to the MI works for the purpose of encouraging food production.

   1. Description about Andhra Pradesh

      Andhra Pradesh state ranked fifth in both area and population of the country. About 75 percent of the states population lives in rural areas and they largely depend on agriculture for their sustenance. The state has a geographical area of 274.4 lakhs ha.

   2. Agriculture Sector in Andhra Pradesh

      The state cultivates a net sown area of 106.4 lakhs ha accounting for 38.8% of the total geographical area of the state. The share of agriculture sector in the gross state domestic product (GSDP) stands at 28.3%. The average annual growth rate of agriculture sector during the last five year period stands at 3.9 percent. Negative growth rate was observed in the years 1994-95 and 1997-98.

   3. Irrigation Sector in Andhra Pradesh

      Aided by 40% of net sown area under irrigation, AP has a cropping intensity of 122 percent. The net irrigated area of 44.5 lakhs ha is contributed from canals (38%) and tanks (17%); and the balance by wells, tube wells and other wells (45%). In the last three decades, net irrigated area has increased from

      29.6 lakhs ha to 41.5 lakhs ha. Canal irrigated area has

      gone up from 13.02 lakhs ha to 15.7 lakhs ha during this period but its share in net irrigated area has come down from 45 to 38%. Tanks with a net irrigated area of 10.7 lakhs ha accounted for 36% of the net irrigated area in triennium ending 1968. But in triennium ending 1998, tanks irrigated only 7.2 lakhs ha accounting for only 17% of the net irrigated area.

      Tanks as a source of irrigation in 1960s through 1990s have therefore, depressed the overall growth in net irrigated area

      Figure 1. Declining trend in area irrigated by tanks in AP, 5-year moving average (000 ha)

      1. Minor Irrigation Sector in Andhra Pradesh

         Minor Irrigation schemes occupy a prominent place in the history of irrigation development in the state of Andhra Pradesh. The state has 12,351 MI sources as of now commanding a total ayacut of 12.5 lakh hectares which is maintained by Irrigation department. In addition to these, there are small tanks commanding an ayacut of less than 40 ha. About 70, 474 such small tanks commanding a total ayacut of over 6 lakh ha is maintained by Panchayat Raj department. During the last decade, only 9,147 MI sources out of the total of 12,351 MI sources were actually functioning in the state indicating that nearly one-fourth of the MI tanks failed to irrigate any area during this period.

      2. Degeneration of MI Tanks

         Degeneration in tank irrigation system is an established trend. This is because of deterioration in the components of these tank systems from the originally designed standards. The affected components are feeder channels, bunds, revetment of bund, sluices, shutters, irrigation canals and surplus courses. As a consequence, they have become inefficient in receiving the due share of waters from the upstream catchment areas, in holding the storage at designed levels at different stages of irrigation or in distributing the waters in the envisaged command areas. To study the performance of these tanks, it is necessary to examine whether the designed inflows are available from the upstream catchment areas for the prevailing spatial and temporal distribution of rainfall. Further it is necessary to examine even in case adequate flows are forthcoming, whether the tanks with ideal conditions of the components, will be able to alter the inflow hydrology to the desired outflow patterns.

      3. Identification of Problem

         In the above context, it is proposed to take up hydrological analysis of a typical minor irrigation tank located in a rainfed area to study its inflow hydrology and desired outflow patterns.

      4. Problem Definition

         For studying the inflow hydrology and desired outflow patterns of an MI tank, it is necessary to carryout hydrological simulation using appropriate model for runoff computation. In the present study, Stranges runoff model is identified and adopted for this purpose.

      5. Study Area

         One minor irrigation tank known as Large tank (Pedda Cheruvu in Telugu) existing in the Chittivalasa village (Latitude 17o 26 10, Longitude 83o 26 10) of Bheemunipatnam mandal in Visakhapatnam district of Andhra Pradesh state, India is selected as the study area. The tank is maintained by Irrigation and Command area development department of Government of Andhra Pradesh.

         The climate in the study area is normally hot and humid. The temperature ranges from 18oC in December to 37oC in May. Sandy loamy soils are present in the study area. The study area is influenced by South-West and North-East monsoons. The study area experiences drought conditions often, as no major irrigation system exists to cushion the vagaries of the monsoon. Hence farmers here mostly depend on the rainfed MI tanks for irrigating their fields.

      6. Objective of the Study

         The objective of the present study is to carryout hydrological simulation for the assessment of physical enefits of a typical rainfed minor irrigation tank located in Chittivalasa village of Bheemunipatnam mandal in Visakhapatnam district of Andhra Pradesh, India.

      7. Scope and Limitations of the Study

         The scope and limitations of the present study are given below.

         1. To carryout hydrological simulation for the assessment of physical benefits of MI tank at Chittivalasa for 30 years from 1978-79 to 2007-08.

         2. The MI tank is assumed to be in ideal conditions

         3. Bheemunipatnam raingauge station is assumed to be the only available influencing raingauge station in the catchment area.

         4. 30 years monthly rainfall data recorded at Bheemunipatnam raingauge station is considered as the basic input

         5. The effective catchment area is calculated by considered 100% of free catchment area and 50% of intercepted catchment area

         6. Stranges runoff model is selected for computing runoff yields.

         7. Two cropping seasons kharif and rabi are considered for assessing benefits

         8. Paddy is the only identified cropping pattern in the study area

         9. Modified penman method is used to calculate the crop water requirement

         10. Appropriate agronomical inputs have been assumed.

      8. Significance of the Study

      The study assumed special significance in the context of assessing the benefits of an MI tank which will be carried out in a systematic manner. For the design of new MI tanks or for taking up rehabilitation measures for the existing MI tanks, it is necessary to study he techno-economic feasibility of the projects before making investments. For carrying out such techno- economic feasibility studies, it is necessary to examine whether sufficient inflows are available in the upstream catchment areas for the prevailing spatial and temporal distribution of rainfall. Further it becomes necessary to examine even incase adequate flows are forthcoming, whether the tanks with ideal conditions of the components will be able to alter the inflow hydrology to the desired outflow patterns.

2. Review of Literature

   Hydrological simulation model may be defined as generalization of an organized methodology based on standard techniques which are repetitive and iterative in nature. A hierarchical scheme for the systematic testing of hydrological simulation models was proposed by V. Klemes [1] in the early 1986.

   Shu-Li Huang and John D. Keenan [2] have developed a deterministic hydrological model by integrating the integral empirical relationships and applied to the Brandywine basin located in south eastern Pennsylvania and northern Delaware in the year 1987.

   Krishna Moan M et al. [3] was the first to devise a hydrological simulation model in the year 1999 for MI tanks based on 75% and 50% dependability rainfall and applied the model for assessing the simulated physical benefits of 384 MI tanks located in various districts of Andhra Pradesh.

   The hydrological simulation model was applied to assess the simulated physical benefits of 2,596 other MI tanks in Andhra Pradesh under APERP in the year 2000 [4].

   During the year 2001, the simulated physical benefits of various MI tanks proposed under APIII were assessed using this hydrological simulation model [5].

   In the present hydrological simulation model, it is proposed to assess the actual year wise simulated physical benefits of MI tanks rather than considering the 75% and 50% dependability rainfall and in that it is an improvement over the previous model developed and applied by Krishna Mohan M et. al. [3], [4], [5].

3. Data Inputs

   Data is collected on Tank Geometry, Rainfall, Pan Evaporation, Potential evapotranspiration values of the study area from various agencies. The collected data is analysed using standard techniques and the inputs for the model were prepared.

   1. Tank Geometry

      The Chittivalasa MI tank is situated in the latitude of 17o 56 10 and longitude of 83o26 10 has the following dimensions. The tank bund is of homogeneous embankment type. The bund has a length of 643 metres. The top width of the bund is 1.2 metres and bottom width is 8 metres. The capacity of the tank at FTL is 0.165 M.cu.m. The waterspread area of the tank at FTL is 0.1020 M. sq.m.

      Table 1: Tank Geometry of Chittivalasa MI tank

      Figure 2. A view of waterspread area of Chittivalasa MI tank

   2. Catchment Area

      The MI tank is identified on the SOI toposheet No 65 O/5 and the catchment area is marked with greater accuracy duly verifying the contour values along the ridges and valleys. The free as well as intercepted catchment areas were marked accordingly.

      The catchment area is measured with the help of planimeter. The free catchment area of the tank is measured as 2.5175 sq.km and intercepted catchment area is found to be 1.42 sq.km. The effective catchment area is worked out using the following formula.

      Effective catchment area = Free catchment area + (20%

      * Intercepted catchment area)

      The effective catchment area comes to around 2.8015 sq. km.

   3. Command Area

      The Registered ayacut of the tank is 40 ha. Usual cropping pattern in the command area is Paddy in both Kharif and Rabi. A part of the command area of the MI tank is shown below.

      Figure 3. A view of command area of Chittivalasa MI tank

      The 3-year moving average shows a maximum value of 1799.9 mm and a minimum value of 701.8 mm, and 5- year moving average shows a maximum value of 1545.94 mm and 836.6 mm. According to Weibulls plotting position, the 75% dependability rainfall works out to be 628 mm and 50% dependability rainfall works out to be 940.6 mm.

      Table 2. Monthly and annual rainfall data in mm

   4. Rainfall

      The monthly rainfall data recorded at Bheemunipatnam raingauge station has been collected for 30 years from 1978-79 to 2007-08. The data is arranged in sequence from June to May as the hydrological year starts from June in the study area. Annual rainfall is computed and presented in the following tables. It is observed that highest annual rainfall is recorded during 1985-86 with a magnitude of 2470.9 mm and the lowest annual rainfall is recorded during 2002-03 with a magnitude of

      473.4 mm. The annual rainfall of above 1000 mm is recorded in another 10 years during 1986-87 (1776.6 mm), 1989-99 (1429.3 mm), 1989-90 (1375.8 mm), 2005-06 (1307.7 mm), 1992-93 (1190.2 mm), 1987-88 (1152.2 mm), 1994-95 (1145.6 mm)1982-83 (1142.7 mm), 1996-97 (1128.2 mm), 2006-07 (1014.5 mm).. Below normal rainfall of less than 800 mm is recorded in another 5 years during 2001-02 (776.6 mm), 1993-94 (693.8), 1999-00 (648.2),1984-85 (628 mm), 2007-08 (605.4 mm.).

      Mean monthly rainfall distribution shows that the study area is receiving most of the rainfall during 5 months starting from June to October in any year. Maximum amount of mean monthly rainfall is observed highest in October followed by August, September, June and July. But the standard deviation is fluctuating from 64.47 in July to 258.13 in August while the coefficient of variation is fluctuating from 0.53 in July to 1.28 in August.

      Figuare 4. Annual Rainfall in mm

      Figure 6. Mean monthly rainfall distribution

      Figure 7. Standard deviation of monthly rainfall

      Figure 8. Coefficient of variation of monthly rainfall

      Table 3. 3-year and 5-year Moving Average

      Figure 5. 3- year and 5-year moving average

      Table 4. Weibulls Plotting Position

      and the Hydrological Simulation has been carried out for 30 years.

      1. Runoff

         Mr. W. L. Strange carried out investigations on catchments in South India and worked out Runoff yields for given rainfall events according to the nature of the catchments. The catchments prone to producing higher yields were categorised as good catchments. The catchments producing low yields are categorized asbad catchments. The intermediate type were called as average catchments. The values of rainfall events and the corresponding runoff events were given in table. The stranges rainfall events and runoff yields were plotted for Good, Average and Bad catchments as shown in figure and an average polynomial relationship of order 2 is approximately established as given below with in the acceptable range of mean square distance. The polynomial relationship of order 2 established between Rainfall in mm to Runoff in M.cu.m for various types of catchments are shown in figures 9 to 13.

         Stranges relationship for good catchments is given by y = 5E-07×2 – 1E-04x + 0.006, R² = 0.998

         Stranges relationship for average catchments is given by

         y = 3E-07×2 – 6E-05x + 0.002, R² = 0.999

         Stranges relationship for bad catchments is given by y = 2E-07×2 – 4E-05x + 0.002, R² = 0.999

         Stranges relationship for catchments with 50% Good and 50% Average conditions is given by

         y = 4E-07×2 – 8E-05x + 0.004, R² = 0.999

4. Methodology and its Application

   Runoff has been computed from the catchment using Stranges runoff table. Crop water requirements were calculated using Modified penman method. Evaporation losses have been appropriately assumed

   Stranges relationship for catchments with 50%

   Average and 50% Bad conditions is given by

   y = 3E-07×2 – 5E-05x + 0.002, R² = 0.999

   The yield rate per sq. km is estimated using the stranges method for the given nature of catchment.

   The yield rates multiplied by the effective catchment area will give rise to inflows during that month.

   Table 5. Stranges runoff yield per sq. km of catchments which are good, average, bad etc.

   Figure 9. Rainfall Runoff yield relationship for Stranges good catchment

   Figure 10. Rainfall Runoff yield relationship for Stranges average catchment

   Figure 11. Rainfall Runoff yield relationship for Strange bad catchment

   Figure 12. Rainfall Runoff yield relationship for Stranges catchment 50% good and 50% average

   Figure 13. Rainfall Runoff yield relationship for Stranges catchment 50% average and 50% bad

   1. Validation of Runoff Yield Rates

      Observed flows are not available for any period during the last 30 years. Hence it is attempted to validate the runoff yield rates obtained from the stranges runoff model with the inflows per sq km of a near by major irrigation project. The variation of computed and observed yield rates is found to be within ±5%.

   2. Crop water requirements

      Two crop seasons are identified in the study area viz. kharif and rabi. The identified cropping pattern in the study area is Paddy only. The modified penman method is used to compute the crop water requirements. The Potential Evapotranspiration values of Ranga Reddy district are collected from the IMD through Irrigation department. The values are given in the following table 6.

      Table 6. Potential Evapotranspiration values of Visakhapatnam district

      The modified penman method is used to compute the crop water requirements. The crop coefficient for paddy is taken as 1.1 for first 3 months and 0.95 for the fourth month in both kharif and rabi. Monthly water requirement in mm is obtained by multiplying the PET value with crop coefficient. A provision of 40 mm for nursery is made during the first month in both kharif and rabi. An allowance of 90 mm for land preparation during the first month is considered in both kharif and rabi. An allowance of 90 mm for four months in both kharif and rabi is provided for deep percolation at the rate of 3 mm per day. An allowance of 50 mm for 2 months is provided for minimum depth in kharif and rabi. After making all the above allowances the gross monthly water requirement is found out in mm. Considering 50% of actual rainfall during the corresponding month as effective rainfall, it is subtracted from the gross monthly water requirement to obtain net irrigation requirement. Assuming 80% field efficiency and 90% conveyance efficiency, the total crop water requirement is found out in mm and subsequently the total requirement per ha in cu.m. is found out. The model calculation of crop water requirements are shown in table 7 and 8 for kharif and rabi respectively.

      Table 7. Model calculation of crop water requirement for the year 1978-79, kharif

      Table 8. Model calculation of crop water requirement for the year 1978-79, rabi

      The crop water requirement for 30 years for both kharif and rabi were computed and presented in table 9 given below.

      Crop water requirement is dependant on various factors like rainfall, crop coefficient and potential evapotranspiration values. The crop water requirement will be high during first month of any season compared to other months due to additional requirement for nursery and land preparation during first month. The crop water requirements are found to be higher in Rabi compared to Kharif due to scanty rainfall during Rabi.

      The maximum value of mean monthly crop water requirement is found during first month of Kharif with a magnitude of 5680 cubic metres per hectare while the lowest value is found during fourth month with a magnitude of 1361 cubic metres per hectares. The standard deviation fluctuated from 966 to during fourth month to 451 during first month of Kharif while the coefficient of variation fluctuated from 0.71 during fourth month to 0.08 during first month.

      The mean monthly crop water requirement is found to be maximum in rabi during the first month with a magnitude of 6130 cubic metres per hectare. The crop water requirement is found to be lowest during second month in Rabi with a magnitude of 2602 cubic metres

      per hectare. However, the standard deviation fluctuated from 92 during first month to 173 during second month while the coefficient of variation fluctuated from 0.01 during first month to 0.07 during second month of Rabi.

      Table 9. Computed monthly crop water requirement for 30 years from 1978-79 to 2007-08, kharif and rabi

      Figure 14. Mean monthly crop water requirement, kharif

      Figure 15. Mean monthly crop water requirement, rabi

      Figure 16. Standard deviation of crop water requirement, kharif

      Figure 17. Standard deviation of crop water requirement, rabi

      Figure 18. Coefficient of variation of crop water requirement, kharif

      Figure 19. Coefficient of variation of crop water requirement, rabi

   3. Evaporation losses

      The monthly pan evaporation data pertaining to Visakhapatnam district is collected from IMD and are presented here. The losses are calculated using the formula given below.

      Average Monthly evaporation losses = (Average Storage / Gross Storage) * Water Spread Area * Pan Evaporation.

      Only 50% of the inflows of June month every year are considered as inflows for June.

      Table 10. Monthly pan evaporation data of Visakhapatnam district

   4. Hydrological Simulation

   After computing the month-wise inflows, the crop water requirements and losses, the end storage during any month is calculated by adding the inflows to the initial storage and subtracting from it the crop water requirement and the losses. If the end storage is greater than the gross capacity of the tank at FTL, then the tank will retain the water up to its gross capacity and the remaining water goes as surplus. If the sum of crop water requirement and losses during any month exceeds the sum of initial storage and inflows, then the difference of two sums will represent deficit for that particular month.

   It is with this mechanism in mind, a simulation exercise has been carried out in MS-Excel package to compute the maximum possible cropping area for each year under kharif and rabi seasons in such a way that there is no deficit and no surplus (or minimum surplus). The model run of the hydrological simulation for one year during 1978-79 is presnted in table 11. It is observed from the simulation run that the tank could irrigate 1 hectare during kharif and 0 hectares during rabi. The end storage of 0.0088 M.cu.m during May of hydrological year 1978-79 will be carry forwarded as e initial storage for the next hydrological year starting with June 1979-80.

   The simulation exercise has been continued for the subsequent 29 hydrological years and each year the simulated irrigated area details are found out and tabulated in table 12 given under results.

   Table 11. Model run of the hydrological simulation for the year 1978-79

5. Results

   The results of the simulation are presented in table 12. The surplus history of the MI tank is presented in table 13.

   Table 12. Results of hydrological simulation of Chittivalasa MI tank

   Figure 20 Registered ayacut and simulated ayacut, kharif

   Figure 21 Registered ayacut and simulated ayacut, rabi

   Table 13. Surplus history of Chittivalasa MI Tank

6. Conclusions

   The results indicated that the MI tank has received sufficient inflows only for 2 years during 1985-86 and 1986-87 to irrigate the entire registered ayacut of 40 ha in kharif. However in rabi, the tank has received inflows to irrigate an ayacut of 8 ha during 186-87 and 5 ha during 1985-86. Except these 2 years, the tank has under performed and the tank has not irrigated more than15 hectares in the remaining 28 years. The tank has received inflows only for 4 years which are sufficient to irrigate an ayacut of 10 to 15 ha during 1990-91 (20

   ha), 1996-97 (12 ha), 2006-07 (12 ha) and 1997-98 (10

   ha).

   The tank has not received sufficient inflows even to irrigate 1 ha during 2002-03 either in kharif or rabi. The tank failed to irrigate even 1 ha during kharif of 2001- 02 but it could irrigate just 1 ha during 2001-02. The tank has received inflows to irrigate just 1 ha only for 2 years during 1978-79 and 1979-80 and 2 ha only for 3

   years during 1991-92, 2001-02 and 2007-08.

   The results indicate that the living conditions of the people whose livelihoods are linked to this Chittivalasa MI tank were pathetic since many years owing to the vagaries of monsoon. It is because of the good efforts and timely intervention of the successive governments in Andhra Pradesh state in terms of providing various drought relief measures and welfare schemes to these people that made them keep going in their routine life.

   Out of the 30 years from 1978-79 to 2007-08, the tank has surplused only on few occasions. The tank has surplused for 8 months during these 30 years. The quantity of surplus water ranges from a low of 0.057 M.cu.m during October, 1986-87 to 2.5941 M.cu.m. during August, 1985-86. The surplus history of the MI tank shows that there exists scope for additional storage if de-silting operations are taken up to increase the capacity of the tank.

   It is always advisable to convert such a rainfed tank in to a system-fed tank by constructing a feeder channel to the tank from a near by major irrigation project.

7. References

1. V. Klemes. Operational testing of hydrological simulation models, Journal of Hydrological Sciences, Vol. 31, Issue 1, pp 13-24, 1986.

2. Shu-Li Huang, John D. Keenan. Hydrological simulation of the Brandywine basin , Journal of the American Water Resources Association, JAWRA, Vol. 23, Issue 3 pp 403- 421, 1987.

3. Krishna Mohan M, Sethurathinam S, Selvarajan S.Techno-Economic Feasibility Study of APERP – Minimum Rehabilitation of 384 selected Minor Irrigation Schemes in Andhra Pradesh I & CAD department, GoAP and AFC Ltd., 1999.

4. Krishna Mohan M et. al. Techno-Economic Feasibility Study of APERPI- Minimum Rehabilitation of 2,596 Minor Irrigation Schemes in Andhra Pradesh I & CAD department, GoAP and AFC Ltd., 2000.

5. Krishna Mohan M et. al. Techno-Economic Feasibility Study of APIII- Minimum Rehabilitation of Minor Irrigation Schemes in Andhra Pradesh I & CAD department, GoAP and WAPCOS(I) Ltd., 2001.