1 00:00:07,399 --> 00:00:08,750 Welcome! 2 00:00:08,750 --> 00:00:12,549 My name is Hubert Savenije and I am a Hydrologist. 3 00:00:12,549 --> 00:00:17,140 Today we are going to talk about evaporation 4 00:00:17,140 --> 00:00:24,140 Of all moisture fluxes over the Earth’s surface, evaporation is the largest outgoing 5 00:00:24,269 --> 00:00:26,080 flux. 6 00:00:26,080 --> 00:00:32,009 Of course precipitation is larger, being the source of all terrestrial moisture. 7 00:00:32,009 --> 00:00:39,009 But in general, evaporation is larger than river discharge, particularly in dry climates. 8 00:00:40,379 --> 00:00:48,519 Many professionals, particularly engineers, consider evaporation a ‘loss’: precious 9 00:00:48,530 --> 00:00:53,010 water that we ‘loose’ to the atmosphere. 10 00:00:53,010 --> 00:00:59,760 This thinking has lead to ambitious projects, such as the proposed draining of the Sudd 11 00:00:59,760 --> 00:01:05,489 wetland in South Sudan, through the Jonglei canal. 12 00:01:05,489 --> 00:01:08,129 But evaporation is not a loss. 13 00:01:08,580 --> 00:01:14,230 Evaporation is responsible for all biomass production through transpiration; 14 00:01:14,930 --> 00:01:21,030 And moreover much of the evaporated moisture returns to the Earth as recycled moisture, 15 00:01:21,030 --> 00:01:22,810 as we have seen earlier. 16 00:01:23,960 --> 00:01:30,960 If we don’t understand evaporation, then we also can’t understand river runoff, because 17 00:01:30,960 --> 00:01:32,890 they are closely intertwined 18 00:01:35,600 --> 00:01:40,420 There are five distinct types of evaporation: 19 00:01:40,430 --> 00:01:46,570 Split in direct and indirect evaporation 20 00:01:46,570 --> 00:01:48,909 Direct evaporation consists of: 21 00:01:48,909 --> 00:01:53,990 Open water evaporation: the evaporation from a free water surface 22 00:01:53,990 --> 00:02:01,290 Soil evaporation: the evaporation of soil moisture reaching the surface through capillary 23 00:02:01,290 --> 00:02:03,290 rise 24 00:02:03,400 --> 00:02:11,540 Interception evaporation: the evaporation of a wet surface after a rainfall event 25 00:02:11,540 --> 00:02:17,620 Sublimation: the evaporation of snow or ice directly into the gas phase 26 00:02:18,859 --> 00:02:22,980 The indirect evaporation is called Transpiration: 27 00:02:22,980 --> 00:02:28,870 the exchange of moisture through the stomata of vegetation 28 00:02:28,870 --> 00:02:35,870 The total evaporation from the land surface is the sum of all these processes. 29 00:02:35,999 --> 00:02:45,719 The first 4 are purely physical processes that transform moisture form the solid or 30 00:02:45,730 --> 00:02:48,439 liquid phase into the gas phase 31 00:02:50,480 --> 00:02:57,480 Transpiration of moisture is a rest product of photosynthesis in vegetation. 32 00:03:00,930 --> 00:03:07,930 Some people call the Total Evaporation “Evapotranspiration” 33 00:03:08,099 --> 00:03:14,950 I advise strongly against it. The term is opaque jargon that nobody understands outside 34 00:03:14,950 --> 00:03:17,489 an incrowd community 35 00:03:17,489 --> 00:03:24,400 It hides the fact that evaporation consists of many different processes 36 00:03:24,400 --> 00:03:32,900 Famous experts on evaporation, such as Brutsaerts and Shuttleworth, avoid the term. 37 00:03:35,240 --> 00:03:40,440 Evaporation is generally constrained by the supply of moisture. 38 00:03:40,449 --> 00:03:47,449 Logical: If there is no moisture available, then there is nothing to evaporate. 39 00:03:47,639 --> 00:03:53,049 We use the term Potential Evaporation for evaporation where there is no shortage of 40 00:03:53,049 --> 00:03:54,749 water, 41 00:03:54,749 --> 00:04:01,699 Or where there are no other limiting factors besides the availability of energy. 42 00:04:01,699 --> 00:04:08,699 The actual evaporation is what you get if you take all limiting factors into account. 43 00:04:10,430 --> 00:04:15,050 In the previous module we looked at the water balance 44 00:04:17,520 --> 00:04:19,520 Here is the water balance: 45 00:04:20,660 --> 00:04:27,840 The change in storage equals to the precipitation, minus the evaporation, 46 00:04:27,840 --> 00:04:29,940 minus the runoff. 47 00:04:30,699 --> 00:04:36,810 The water balance closely connects the precipitation, the evaporation and the runoff. 48 00:04:36,810 --> 00:04:45,850 If we now look at averages, if we take the average over the year, over many years, 49 00:04:46,280 --> 00:04:52,840 then this term becomes very small, and we may often disregard it, 50 00:04:52,840 --> 00:04:57,300 if the average is taken over several years. 51 00:04:57,479 --> 00:05:04,479 The total amount of rainfall that has fallen over, for instance, 10 years is, of course, 52 00:05:05,069 --> 00:05:12,069 much larger than the difference in storage between the beginning and the end of the decade. 53 00:05:13,569 --> 00:05:17,900 The same can be said about the other fluxes. 54 00:05:17,900 --> 00:05:23,900 So for long-term averages, indicated by a bar above the variables, 55 00:05:23,900 --> 00:05:27,640 a very simple relation applies: 56 00:05:27,650 --> 00:05:34,890 Whereby the evaporation equals the difference between the precipitation and the discharge 57 00:05:35,320 --> 00:05:39,859 (all expressed per unit surface area) 58 00:05:40,880 --> 00:05:49,360 Dividing by the precipitation we see that the proportion of evaporated precipitation 59 00:05:49,360 --> 00:05:54,909 equals 1 minus the runoff coefficient. 60 00:05:54,909 --> 00:06:00,610 So there are two conditions that apply to evaporation: 61 00:06:00,610 --> 00:06:07,610 The actual evaporation E is always less than the average annual potential evaporation 62 00:06:08,840 --> 00:06:15,840 And the average annual evaporation should be less than the precipitation. 63 00:06:16,219 --> 00:06:19,449 The first condition is an energy constraint 64 00:06:19,449 --> 00:06:22,669 The second is a moisture constraint. 65 00:06:22,669 --> 00:06:28,469 These conditions form the asymptotes of the Budyko curve 66 00:06:31,480 --> 00:06:39,320 Budyko (1920-2001) developed a simple mathematical form to describe the relation between average 67 00:06:39,330 --> 00:06:44,430 annual evaporation, precipitation, and potential evaporation 68 00:06:46,580 --> 00:06:51,420 There are many mathematical forms, but this is the simplest. 69 00:06:52,600 --> 00:06:59,600 For the Budyko curve we have a relation between the [...], 70 00:06:59,600 --> 00:07:06,140 which is defined by the potential evaporation divided by the rainfall, 71 00:07:06,140 --> 00:07:13,220 and these are average annual values, and this [...] is 1 in this point, 72 00:07:13,300 --> 00:07:17,340 if you are on the left of 1, you are in a wet climate, 73 00:07:17,340 --> 00:07:20,620 on the right you are in a dry climate. 74 00:07:21,050 --> 00:07:27,130 On the vertical axis we have the relative evaporation, defined by: 75 00:07:27,130 --> 00:07:32,290 the average annual evaporation divided by the avarage annual precipitation. 76 00:07:32,290 --> 00:07:39,130 And of course this is bound by a maximum 1, which you could say is the moisture constraint. 77 00:07:39,130 --> 00:07:43,110 You can never have more evaporation than precipitation. 78 00:07:43,110 --> 00:07:48,970 And on the other hand, we have what we call the energy constraint, 79 00:07:48,970 --> 00:07:52,790 that means that the evaporation can never be more than the potential. 80 00:07:52,790 --> 00:07:56,890 And the Budyko curve nicely runs between them. 81 00:07:56,890 --> 00:08:05,710 And the distance between the moisture constraint and the curve, this is the runoff coefficient. 82 00:08:06,380 --> 00:08:11,821 Here we plotted the water balance of some of the larger river basins of the world within 83 00:08:11,830 --> 00:08:13,830 the Budyko diagram. 84 00:08:13,830 --> 00:08:23,370 The wet catchments (Amazon, Parana, Ganges and Yangtze have a less than 1 aridity 85 00:08:23,520 --> 00:08:29,500 and have a large runoff coefficient (more than 40%) 86 00:08:29,509 --> 00:08:36,509 The others lie in dryer territories and have a smaller runoff coefficient. 87 00:08:37,090 --> 00:08:39,820 The plots don’t lie exactly on the curve. 88 00:08:39,820 --> 00:08:46,260 There may be many reasons for that, among them errors of observation or calculation, 89 00:08:46,260 --> 00:08:55,040 But there are also physical reasons, related to landscape, geology, climate and vegetation. 90 00:08:55,870 --> 00:08:59,720 But this goes a bit beyond the scope of this lecture. 91 00:08:59,720 --> 00:09:04,240 If you are interested, there is a lot of literature that you can refer to. 92 00:09:04,240 --> 00:09:10,260 There are 4 meteorological factors that affect evaporation: 93 00:09:10,260 --> 00:09:11,580 The energy balance 94 00:09:11,580 --> 00:09:14,910 driven by incoming solar radiation 95 00:09:14,910 --> 00:09:17,320 The humidity of the air 96 00:09:17,320 --> 00:09:21,210 And the aerodynamic resistance 97 00:09:21,210 --> 00:09:24,670 We start with the radiation balance 98 00:09:24,670 --> 00:09:36,020 The Net incoming short wave radiation is the amount of short wave radiation entering the top of 99 00:09:36,020 --> 00:09:40,480 the atmosphere: RA 100 00:09:40,480 --> 00:09:46,980 Minus the amount that is reflected or absorbed by clouds: Rc 101 00:09:46,980 --> 00:09:53,980 Minus the part that is reflected by the Earth surface (r Rc) 102 00:09:55,580 --> 00:10:02,120 Minus the part of the energy that is lost by long wave radiation: RB 103 00:10:02,130 --> 00:10:07,660 The reflectivity is called the albedo or the whiteness. 104 00:10:07,660 --> 00:10:13,370 A completely black surface has an albedo of zero. 105 00:10:13,370 --> 00:10:16,870 It absorbs all radiation 106 00:10:16,870 --> 00:10:23,870 Snow has an albedo close to 1, reflecting almost all incoming radiation 107 00:10:25,840 --> 00:10:34,880 We can measure the incoming radiation Rc and the outgoing radiation fluxes r Rc and RB 108 00:10:34,880 --> 00:10:38,300 With a Radiometer 109 00:11:35,600 --> 00:11:41,940 We can also use a more classical instrument to determine the number of hours of sunshine 110 00:11:41,940 --> 00:11:46,560 The Campbell-Stoke sunshine recorder. 111 00:11:46,560 --> 00:11:51,930 Depending on where you are on Earth there are different empirical formulas to calculate 112 00:11:51,930 --> 00:11:58,930 Rc as a function of the number of sunshine hours and short wave radiation RA. 113 00:12:00,040 --> 00:12:05,930 The potential number of hours depends on the time of the year and the position on Earth 114 00:12:05,930 --> 00:12:12,930 This is tabulated and so is the short wave radiation occurring above the clouds. 115 00:12:13,610 --> 00:12:20,000 The outgoing long wave radiation RB is also calculated with a semi-empirical equation, 116 00:12:20,000 --> 00:12:25,140 based on the long wave radiation of a warm body. 117 00:12:25,140 --> 00:12:34,680 The left part of the equations contains the Stefan Bolzmann coefficient sigma and t_a 118 00:12:34,680 --> 00:12:37,840 is the actual temperature near the surface. 119 00:12:37,840 --> 00:12:47,100 The right part contains the effect of clouds, which reduce the outgoing radiation like a 120 00:12:47,110 --> 00:12:49,090 blanket. 121 00:12:49,090 --> 00:12:57,490 The middle part contains the air humidity (indicate e_a), water vapour being a strong 122 00:12:57,490 --> 00:13:00,240 greenhouse gas. 123 00:13:00,240 --> 00:13:06,200 So if the air is dry, there is more outgoing long wave radiation. 124 00:13:06,200 --> 00:13:09,940 Other greenhouse gasses are not included in this equation. 125 00:13:09,940 --> 00:13:16,940 But humidity is, because it is a strong and highly variable greenhouse gas 126 00:13:18,200 --> 00:13:22,990 This brings us to the humidity, which not only affects the long wave radiation, but 127 00:13:22,990 --> 00:13:26,740 also directly influences evaporation. 128 00:13:26,740 --> 00:13:33,740 Let’s first look at the relation for Saturated Vapour Pressure. 129 00:13:33,860 --> 00:13:39,500 On the horizontal axis is the temperature and on the vertical axis the pressure at which 130 00:13:39,500 --> 00:13:46,410 water vapour is saturated in the air and reaches the point of condensation. 131 00:13:46,410 --> 00:13:54,970 This is called the saturation vapour pressure, expressed in kPa or kN/m2 132 00:13:56,260 --> 00:14:01,260 The equation can be derived theoretically from Entropy considerations 133 00:14:01,260 --> 00:14:04,930 The slope of the curve is s 134 00:14:04,930 --> 00:14:10,690 How do we get onto the curve. 135 00:14:10,690 --> 00:14:18,800 Imagine we are in the point P with temperature ta and saturation pressure ea, reflecting 136 00:14:18,800 --> 00:14:23,180 the number of moisture molecules per unit volume. 137 00:14:23,180 --> 00:14:29,500 If we now move left in the figure until we hit the curve, we have reached the dew point 138 00:14:29,500 --> 00:14:35,030 td, where the molecules start to condense. 139 00:14:35,030 --> 00:14:44,490 We can also move up at the same temperature, but now we are increasing the number of molecules 140 00:14:44,490 --> 00:14:49,130 until the pressure is so high that they condense. 141 00:14:49,130 --> 00:14:55,900 There also is a spontaneous way to reach the curve, if we have a wet surface, like a wet 142 00:14:55,900 --> 00:14:58,810 sponge 143 00:14:58,810 --> 00:15:05,810 If you blow wind over a wet sponge, the fast molecules will leave, the sponge gets cooler, 144 00:15:06,130 --> 00:15:14,360 until it reaches the dew point, but at a higher vapour pressure because it has absorbed energy 145 00:15:14,360 --> 00:15:21,070 from the surrounding (warmer) air. 146 00:15:21,070 --> 00:15:24,680 This equation shows how it happens. 147 00:15:24,680 --> 00:15:31,430 It can be demonstrated by the psychrometer, which has a wet and dry bulb thermometer 148 00:15:31,430 --> 00:15:43,040 We need the psychrometric constant gamma to be able to calculate the actual vapour pressure 149 00:15:43,040 --> 00:15:50,040 and hence the relative humidity. 150 00:16:18,920 --> 00:16:24,200 The energy balance states that the change of energy stored in the ground equals the 151 00:16:24,200 --> 00:16:29,970 difference between the net incoming radiation, the sensible heat flux H, 152 00:16:29,970 --> 00:16:38,530 the advected heat A and the latent heat of evaporation rho-lambda-E 153 00:16:39,360 --> 00:16:46,360 All energy terms are in W/m2, or J/s/m2 154 00:16:46,810 --> 00:16:53,810 We may assume that average over a day the change of energy stored is negligible and 155 00:16:54,280 --> 00:16:58,700 we generally also neglect advection. 156 00:16:58,700 --> 00:17:05,600 This results in an expression for the evaporation depending on the balance between net incoming 157 00:17:05,600 --> 00:17:09,250 radiation and the sensible heat flux. 158 00:17:09,250 --> 00:17:15,150 This is the basis for the Penman equation 159 00:17:15,150 --> 00:17:21,160 Besides the energy balance, it makes use of empirical relations 160 00:17:21,160 --> 00:17:25,110 It uses 4 standard meteorological variables: 161 00:17:25,110 --> 00:17:32,110 air temperature, relative humidity, wind velocity and net radiation 162 00:17:32,690 --> 00:17:40,810 Penman assumed that the sensible heat flux H is correlated to the latent heat flux (evaporation). 163 00:17:41,940 --> 00:17:48,940 He could then rework the equation to obtain the equation for open water evaporation Eo 164 00:17:48,950 --> 00:17:55,950 We recognise the symbols that we have already seen: 165 00:17:56,820 --> 00:18:01,540 the slope of the saturation pressure curve s 166 00:18:01,540 --> 00:18:08,540 The net incoming radiation RN, the density of water rho 167 00:18:09,660 --> 00:18:16,270 the energy required to evaporate a kg of water lambda 168 00:18:16,270 --> 00:18:23,270 the specific heat of air, the difference between the saturation and actual vapour pressure 169 00:18:24,670 --> 00:18:37,790 and the aerodynamic resistance, which is the driver of the turbulence that exchanges air 170 00:18:37,790 --> 00:18:40,960 with air layers higher up. 171 00:18:40,960 --> 00:18:47,960 It exchanges heat and moisture with the atmosphere through diffusive transport 172 00:18:49,590 --> 00:18:56,010 Monteith expanded the open water equation of Penman to a vegetated area. 173 00:18:56,010 --> 00:19:02,780 He introduced the crop resistance rc, which provides a brake on the transpiration of vegetation 174 00:19:02,780 --> 00:19:05,400 as a result of environmental constraints 175 00:19:07,220 --> 00:19:14,220 This crop resistance depends on the opening of stomata in the leaves, which reacts to 176 00:19:14,230 --> 00:19:17,350 the availability of moisture in the soil 177 00:19:17,350 --> 00:19:19,770 the relative humidity 178 00:19:19,770 --> 00:19:22,670 the sunlight for photosynthesis 179 00:19:22,670 --> 00:19:25,150 and the temperature : 180 00:19:25,150 --> 00:19:29,620 Plants don’t like it when it is too hot or too cold. 181 00:19:29,620 --> 00:19:36,060 The details of this go beyond this lecture, but it is not a bad idea to read the paper 182 00:19:36,060 --> 00:19:40,480 by Lan Wang who explains this in detail. 183 00:19:40,480 --> 00:19:45,080 here you see his paper, it's open access. 184 00:19:45,090 --> 00:19:52,090 There are different ways in which components of evaporation can be measured directly: 185 00:19:52,130 --> 00:20:02,150 At catchment scale, it can be done on basis of the water balance: E=P-Q- dS/dt 186 00:20:02,150 --> 00:20:09,770 But there are also instruments, such as the evaporation Pan, the Lysimeter or the shallow 187 00:20:09,770 --> 00:20:12,590 lysimeter 188 00:20:12,590 --> 00:20:17,820 The pan is the most simple way of measuring the open water evaporation as a function of 189 00:20:17,820 --> 00:20:20,880 the atmospheric conditions. 190 00:20:20,880 --> 00:20:27,230 Through an empirical equation the open water evaporation can be derived form the observed 191 00:20:27,230 --> 00:20:29,570 pan evaporation 192 00:20:29,570 --> 00:20:36,570 But open water evaporation is not the same as the evaporation of a vegetated surface 193 00:20:37,640 --> 00:20:48,080 The lysimeter (these can be very large things ranging from 5 to 100 m2 or even more) 194 00:20:48,090 --> 00:20:55,090 is a way of calculating the total evaporation of a vegetated surface by weighing the storage 195 00:20:55,180 --> 00:21:02,180 and of course tracking all the water that goes in, or is removed. 196 00:21:03,300 --> 00:21:08,620 For the measuring of interception we have developed our own instruments. One of our 197 00:21:08,620 --> 00:21:14,290 students developed a devise to weigh a tree, 198 00:21:14,290 --> 00:21:20,550 and Dr Coenders developed a shallow lysimeter to determine the ground interception, 199 00:21:20,550 --> 00:21:23,650 which is published in this open access journal. 200 00:21:25,160 --> 00:21:32,160 It has two containers on top of each other which are continuously weighed 201 00:21:33,130 --> 00:21:38,680 The top one is permeable the lower one has a tap. 202 00:21:38,680 --> 00:21:45,420 From the water balance we can compute the evaporation 203 00:21:45,420 --> 00:21:51,090 More advanced observations of evaporation are done by eddy covariance on a tower, or 204 00:21:51,090 --> 00:21:54,250 by an instrument we developed ourselves: 205 00:21:54,250 --> 00:21:59,130 The DTS based wet and dry cable approach, 206 00:21:59,130 --> 00:22:02,510 which is also published in open access. 207 00:22:02,880 --> 00:22:08,840 You can see that evaporation has many different aspects and that there still is a lot to investigate 208 00:22:08,840 --> 00:22:12,130 before we fully understand. 209 00:22:12,130 --> 00:22:18,750 All the intricate ways in which water changes phase (and face) in the hydrological cycle. 210 00:22:18,750 --> 00:22:25,750 But being difficult to grasp does not mean that it is unimportant. 211 00:22:26,890 --> 00:22:32,830 It still is the largest flux resulting from terrestrial precipitation. 212 00:22:32,830 --> 00:22:37,710 Which reminds me: we have not yet discussed precipitation much. 213 00:22:38,000 --> 00:22:42,480 I’ll see you in the next module, about precipitation.