Zigbee communication outdoors at long distance ( around 1 - 1.5 km)
Minimum data rate for Zigbee is 20k bits per second and this dictates the bandwidth of the receiver. The wider the bandwidth of the receiver, the more noise the receiver picks up so running at a smaller bandwidth means that for a given transmit power, the operating distance to the receiver can be bigger.

Received power needed for a given data rate is: -

Power required in dBm is -154dBm + 10\$log_{10}\$(data rate) dBm

So at 20kbits/sec the power needed is -111 dBm but this is really the hard limit and you should always consider what is called as fade-margin - basically add at least 20dB to this figure - this takes you to -91dBm.

The transmitter, in free space (ideal conditions) pushes power from its antenna and the Friis formula that equates power to distance is this: -

Link Loss (dB) = 32.4 + 20\$log_{10}\$(F) + 20\$log_{10}\$(d) 

where F is MHz and d is distance between the two antennas (kilo metres). 

So if the radio uses 1GHz and was 1.5 km away, the link loss would be 32.4 + 60 + 3.52 = about 96dB. If it's transmitting +10dBm then the power received would be -86dBm. This seems to work because I calculated a 20kbps receiver needed -91dBm BUT again this is a free-space perfect transmission and there are various rules of thumb needed to convert this to "on-earth" with lots of obstacles (this is a very big subject). If you added another 20 to 40dB to these figures you'd be looking at something that should work reasonably reliably.

This means you might need to be transmitting at +40dBm (10 W output power). This is too high for Zigbee so either consider directional antennas that add gain at both transmitter and receiver or consider a mesh network as recommended in the wikipedia link in the question.