[navsal123]

I have tried everything to remove the error, but it still doesnt go. The error comes in the last do loop.

program main
implicit none
integer::i,nnodes,time,tmax,iter,iconv,ib
real::satk,alpha,en,em,thetas,thetar,slength,delt, psiini,psitop,psibot,delz,epsilon,ndays,n2days,z(5 00),psinew(500),psiold(500),psiass(500),A(500),B(5 00)C(500),D(500),psifinal(500),thetaavail(500),the tazero(500),esatavail(500),thetares(500),esatzero( 500),kodeday(500),psires(500)
open(1,file='main.dat',status='new')

print *, "Enter the values of satk,alpha,en,thetas,thetar"
read *, satk,alpha,en,thetas,thetar
write(1,*) "Saturated conductivity=",satk,"Alpha=",alpha,"n=",en,"Satura ted MC =",thetas,"Residual MC =",thetar
em=(1-(1/en))
write(1,*) "Van genuchten m =",em
print *, "Enter the values of soil length, nnodes, delta T, Maximum time"
read *, slength,nnodes,delt,tmax
write(1,*) "Soil length=",slength,"Number of nodes=",nnodes,"Time interval=",delt,"Maximum time =",tmax
delz=(slength)/(nnodes-1)
write(1,*) "Node spacing is =",delz
print *, "Enter the values of initial, bottom, top pressure heads"
read *, psiini,psibot,psitop
write(1,*) "Initial pressure head=",psiini,"Bottom pressure head=",psibot,"Top pressure head=",psitop
print *, "Enter the values epsilon,ndays,n2days"
read *, eepsilon,ndays,n2days
write(1,*) "Prestipulated convergence factor=",eepsilon,"Number days simulation runs=",ndays,"hours of ndays=",n2days
do i=1,nnodes
z(i)=(nnodes-(i))*delz
write(1,*) "Node spacing at node number",(nnodes+1)-i, "is=",z(i)
enddo
print *, "Which boundary condition you want to apply?"
print *, "Enter 1 for Dirichlet boundary condition at top and bottom"
print *, "Enter 2 for flux at top and gravity drainage at bottom"
print *, "Enter 3 for Dirichlet at top and gravity drainage bottom"
print *, "Enter 4 for flux at top and Dirichlet at bottom"
read *, ib

if(ib.eq.1)then
do i=1,nnodes
if(i.eq.1)then
psiold(i)=psibot
elseif(i.eq.nnodes)then
psiold(i)=psitop
else
psiold(i)=psiini
endif
enddo
else
do i=1,nnodes
if(i.eq.1)then
psiold(i)=psibot
else
psiold(i)=psitop
endif
write(1,*) "Initial value of pressure head at",(nnodes+1)-i,"is",psiold(i)
enddo
endif

do i=1,nnodes
esatzero(i)=(1/(1+abs((psiold(i)*alpha))**en)**em)
thetazero(i)=(((esatzero(i))*(thetas-thetar))+thetar)
write(1,*) "thetazero=",thetazero(i),"esatzero=",esatzero (i)
enddo

close(1)
end program main

[FMDenaro]

please, write the compiler error you get

[DarylMusashi]

At first:
print *, "Enter the values epsilon,ndays,n2days"
read *, eepsilon,ndays,n2days

But you defined epsilon, not eepsilon.

Secondly the problem might be the exponential operation in the last do-loop. FORTRAN is not able to raise a negative number to a fractional power.